diff --git a/packages/compiler/src/backend/llvm/checked-value-lifetimes.ts b/packages/compiler/src/backend/llvm/checked-value-lifetimes.ts
index 5f8119079..4169e86d8 100644
--- a/packages/compiler/src/backend/llvm/checked-value-lifetimes.ts
+++ b/packages/compiler/src/backend/llvm/checked-value-lifetimes.ts
@@ -5,6 +5,8 @@
 export function preservesDynTest(test: string): boolean {
   switch (test) {
     case "truthy":
+    case "classCapsule":
+    case "funcKind":
     case "nullish":
     case "promise":
     case "buffer":
diff --git a/packages/compiler/src/backend/llvm/expr-dynamic.ts b/packages/compiler/src/backend/llvm/expr-dynamic.ts
index 434435eae..07a538184 100644
--- a/packages/compiler/src/backend/llvm/expr-dynamic.ts
+++ b/packages/compiler/src/backend/llvm/expr-dynamic.ts
@@ -719,6 +719,10 @@ export function emitDynamicExpr(
         B.line(
           `${test} = call zeroext i1 @scr_dyn_typed_array_is(ptr ${d.name}, i32 ${BYTES_ELEM_NUM[e.bytesElem ?? "u8"]})`,
         );
+      } else if (e.test === "classCapsule") {
+        host.declare(`declare zeroext i1 @scr_dyn_is_class_capsule(ptr)`);
+        test = B.tmp();
+        B.line(`${test} = call zeroext i1 @scr_dyn_is_class_capsule(ptr ${d.name})`);
       } else if (e.test === "truthy") {
         host.declare(`declare zeroext i1 @scr_dyn_truthy(ptr)`);
         test = B.tmp();
@@ -839,6 +843,7 @@ export function emitDynamicExpr(
             undefined: DYN_KIND.UNDEF,
             null: DYN_KIND.NULL,
             bytes: DYN_KIND.BYTES,
+            funcKind: DYN_KIND.FUNC,
           };
           test = oneOf([kindOf[e.test]!]);
         }
diff --git a/packages/compiler/src/backend/llvm/lib-shared.ts b/packages/compiler/src/backend/llvm/lib-shared.ts
index c5145dd2e..e64bc48af 100644
--- a/packages/compiler/src/backend/llvm/lib-shared.ts
+++ b/packages/compiler/src/backend/llvm/lib-shared.ts
@@ -1553,6 +1553,7 @@ export const LIB_FN_SYMS: Record<string, string> = {
   "dyn.assignPrototype": "scr_dyn_assign_prototype",
   "dyn.definePrototypeProps": "scr_dyn_define_prototype_props",
   "dyn.bagGet": "scr_dyn_bag_get",
+  "dyn.bagCreateField": "scr_dyn_bag_create_field",
   "dyn.bagSet": "scr_dyn_bag_set",
   "dyn.objCreate": "scr_dyn_obj_create",
   "dyn.nameClassPrototype": "scr_dyn_name_class_prototype",
diff --git a/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.test.ts b/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.test.ts
index b717452c2..1a21a6aa5 100644
--- a/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.test.ts
+++ b/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.test.ts
@@ -458,9 +458,11 @@ test("normalizes base capsules before dispatching each derived member once", ()
   });
   expect(dispatch.body.filter((statement) => statement.kind === "if")).toHaveLength(2);
   const normalize = liftedFns.find((helper) => helper.name === "%dyn.class.normalize")!;
+  // The capsule branch unboxes its receiver once, then tests each subclass.
   expect(normalize.body[0]).toMatchObject({
     kind: "if",
     then: [
+      { kind: "varDecl", init: { kind: "dynCheck", type: { kind: "object", className: "Base" } } },
       {
         kind: "if",
         then: [
diff --git a/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.ts b/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.ts
index a68aed5b4..b58dcae51 100644
--- a/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.ts
+++ b/packages/compiler/src/frontend/lowering/class-dynamic-dispatch.ts
@@ -394,6 +394,40 @@ export function classInstanceOf(
   };
 }
 
+/** `value` is the parameter itself, possibly through js.fromDyn. */
+function returnsParam(value: IrExpr, param: string): boolean {
+  if (value.kind === "varRef") return value.localId === param;
+  return (
+    value.kind === "libCall" &&
+    value.fn === "js.fromDyn" &&
+    value.args.length === 1 &&
+    value.args[0]!.kind === "varRef" &&
+    value.args[0]!.localId === param
+  );
+}
+
+/** The kind a class-table branch requires of `param`: a capsule-key test
+ * (alone or guarding a conjunction) needs a capsule, a class-key test a
+ * function value; null for any other condition. */
+function branchKind(cond: IrExpr, param: string): "classCapsule" | "funcKind" | null {
+  if (
+    cond.kind === "libCall" &&
+    (cond.fn === "dyn.typedRefIs" ||
+      cond.fn === "dyn.typedRefKeyIs" ||
+      cond.fn === "dyn.classIs") &&
+    cond.args[0]?.kind === "varRef" &&
+    cond.args[0].localId === param
+  ) {
+    if (cond.fn === "dyn.classIs") return "funcKind";
+    const key = cond.args[1];
+    return key?.kind === "strLit" && key.value.startsWith("object:") ? "classCapsule" : null;
+  }
+  if (cond.kind === "ternary" && cond.else_.kind === "boolLit" && cond.else_.value === false)
+    return branchKind(cond.cond, param);
+  if (cond.kind === "logical" && cond.op === "&&") return branchKind(cond.left, param);
+  return null;
+}
+
 const READ_PROPERTIES = "%dyn.class.readProperties";
 const READ_PROPERTIES_OR_EMPTY = "%dyn.class.readPropertiesOrEmpty";
 
@@ -685,16 +719,154 @@ export function trackClassFieldCreation(lowerer: Lowerer, functions: IrFunction[
   if (![...lowerer.classes.values()].some((info) => info.def.tracksOwnFields)) return;
   const loc = functions[0]!.loc;
   const name = "%dyn.class.fieldCreated";
-  const value = varRef("value", DYN, loc),
-    key = varRef("key", DYN, loc);
-  const bag = varRef("bag", DYN, loc);
-  const created = (receiver: IrExpr, className: string, field: string, loc: SrcLoc): IrStmt => ({
+  const key = varRef("key", DYN, loc);
+  // One helper per class: the instance's bag is its own hidden field, so a
+  // creation reads it directly instead of boxing the instance and testing
+  // it against every class's capsule (%dyn.class.properties), which runs
+  // only to create the bag, once per instance.
+  const perClass = new Map<string, string>();
+  const helperFor = (className: string): string => {
+    const known = perClass.get(className);
+    if (known !== undefined) return known;
+    const helperName = `${name}:${className}`;
+    perClass.set(className, helperName);
+    const self = varRef("this", { kind: "object", className }, loc);
+    const bagRef = varRef("bag", DYN, loc);
+    functions.push({
+      name: helperName,
+      params: [
+        { localId: "this", name: "this", type: { kind: "object", className } },
+        { localId: "key", name: "key", type: DYN },
+      ],
+      returnType: VOID,
+      locals: [
+        { id: "this", name: "this", type: { kind: "object", className }, mutable: false },
+        { id: "key", name: "key", type: DYN, mutable: false },
+        { id: "bag", name: "bag", type: DYN, mutable: true },
+      ],
+      body: [
+        {
+          kind: "varDecl",
+          localId: "bag",
+          init: { kind: "fieldGet", obj: self, className, field: PROPERTY_BAG, type: DYN, loc },
+          loc,
+        },
+        {
+          kind: "if",
+          cond: { kind: "dynTest", test: "undefined", value: bagRef, type: BOOL, loc },
+          then: initializeBag(lowerer, className, self, loc),
+          else_: null,
+          loc,
+        },
+        ...defineIfAbsent(lowerer, bagRef, key, loc),
+      ],
+      loc,
+    });
+    return helperName;
+  };
+  // String-keyed fields take the runtime's direct insert for an ordinary
+  // bag; the general path stays for any other bag and for symbol keys.
+  const stringHelperFor = (className: string): string => {
+    const known = perClass.get(`str:${className}`);
+    if (known !== undefined) return known;
+    const general = helperFor(className);
+    const helperName = `${name}.string:${className}`;
+    perClass.set(`str:${className}`, helperName);
+    const type: IrType = { kind: "object", className };
+    const self = varRef("this", type, loc);
+    const bagRef = varRef("bag", DYN, loc);
+    const field = varRef("field", STRING, loc);
+    functions.push({
+      name: helperName,
+      params: [
+        { localId: "this", name: "this", type },
+        { localId: "field", name: "field", type: STRING },
+      ],
+      returnType: VOID,
+      locals: [
+        { id: "this", name: "this", type, mutable: false },
+        { id: "field", name: "field", type: STRING, mutable: false },
+        { id: "bag", name: "bag", type: DYN, mutable: true },
+      ],
+      body: [
+        {
+          kind: "varDecl",
+          localId: "bag",
+          init: { kind: "fieldGet", obj: self, className, field: PROPERTY_BAG, type: DYN, loc },
+          loc,
+        },
+        {
+          kind: "if",
+          cond: { kind: "dynTest", test: "undefined", value: bagRef, type: BOOL, loc },
+          then: initializeBag(lowerer, className, self, loc),
+          else_: null,
+          loc,
+        },
+        {
+          kind: "if",
+          cond: {
+            kind: "libCall",
+            fn: "dyn.bagCreateField",
+            args: [bagRef, field],
+            type: BOOL,
+            loc,
+          },
+          then: [{ kind: "return", value: null, loc }],
+          else_: null,
+          loc,
+        },
+        {
+          kind: "exprStmt",
+          expr: {
+            kind: "call",
+            callee: general,
+            args: [self, lowerer.coerceToExpected(field, DYN)],
+            type: VOID,
+            loc,
+          },
+          loc,
+        },
+      ],
+      loc,
+    });
+    return helperName;
+  };
+  const created = (receiver: IrExpr, className: string, field: string, loc: SrcLoc): IrStmt => {
+    const symbolKey = symbolMemberKey(lowerer, lowerer.classes.get(className)!, field, loc);
+    if (symbolKey === null) {
+      return {
+        kind: "exprStmt",
+        expr: {
+          kind: "call",
+          callee: stringHelperFor(className),
+          args: [
+            receiver.type.kind === "object" && receiver.type.className !== className
+              ? { kind: "upcast", value: receiver, type: { kind: "object", className }, loc }
+              : receiver,
+            { kind: "strLit", value: field, type: STRING, loc },
+          ],
+          type: VOID,
+          loc,
+        },
+        loc,
+      };
+    }
+    return createdGeneral(receiver, className, field, loc);
+  };
+  const createdGeneral = (
+    receiver: IrExpr,
+    className: string,
+    field: string,
+    loc: SrcLoc,
+  ): IrStmt => ({
     kind: "exprStmt",
     expr: {
       kind: "call",
-      callee: name,
+      callee: helperFor(className),
       args: [
-        lowerer.coerceToExpected(receiver, DYN),
+        receiver.type.kind === "object" && receiver.type.className !== className
+          ? { kind: "upcast", value: receiver, type: { kind: "object", className }, loc }
+          : receiver,
         symbolMemberKey(lowerer, lowerer.classes.get(className)!, field, loc) ??
           lowerer.coerceToExpected({ kind: "strLit", value: field, type: STRING, loc }, DYN),
       ],
@@ -769,80 +941,103 @@ export function trackClassFieldCreation(lowerer: Lowerer, functions: IrFunction[
       },
     });
   }
-  functions.push({
-    name,
-    params: [
-      { localId: "value", name: "value", type: DYN },
-      { localId: "key", name: "key", type: DYN },
-    ],
-    returnType: VOID,
-    locals: [
-      { id: "value", name: "value", type: DYN, mutable: false },
-      { id: "key", name: "key", type: DYN, mutable: false },
-      { id: "bag", name: "bag", type: DYN, mutable: false },
-    ],
-    body: [
+}
+
+/** `bag = <the class's new bag>`, stored in the instance's hidden field:
+ * the class's own initializer when recorded, else the class-table
+ * dispatch, which creates it. */
+function initializeBag(lowerer: Lowerer, className: string, self: IrExpr, loc: SrcLoc): IrStmt[] {
+  const init = lowerer.classBagInitializers?.get(className);
+  if (init === undefined) {
+    return [
       {
-        kind: "varDecl",
+        kind: "assign",
         localId: "bag",
-        init: {
+        value: {
           kind: "call",
           callee: classPropertiesHelper(lowerer, loc).name,
-          args: [value],
+          args: [lowerer.coerceToExpected(self, DYN)],
           type: DYN,
           loc,
         },
         loc,
       },
-      {
-        kind: "if",
-        cond: {
-          kind: "unary",
-          op: "!",
-          operand: { kind: "libCall", fn: "dyn.hasOwnComputed", args: [bag, key], type: BOOL, loc },
-          type: BOOL,
-          loc,
-        },
-        then: [
-          {
-            kind: "exprStmt",
-            expr: {
-              kind: "libCall",
-              fn: "dyn.defineProperty",
-              args: [
-                bag,
-                lowerer.coerceToExpected(key, DYN),
-                {
-                  kind: "dynObjLit",
-                  fields: [
-                    {
-                      key: { kind: "strLit", value: "value", type: STRING, loc },
-                      value: dynUndefinedExpr(loc),
-                    },
-                    ...["writable", "enumerable", "configurable"].map((key) => ({
-                      key: { kind: "strLit" as const, value: key, type: STRING, loc },
-                      value: lowerer.coerceToExpected(
-                        { kind: "boolLit", value: true, type: BOOL, loc },
-                        DYN,
-                      ),
-                    })),
-                  ],
-                  type: DYN,
-                  loc,
-                },
-              ],
-              type: DYN,
-              loc,
-            },
-            loc,
-          },
-        ],
-        else_: null,
+    ];
+  }
+  return [
+    {
+      kind: "fieldSet",
+      obj: self,
+      className,
+      field: PROPERTY_BAG,
+      value: structuredClone(init),
+      loc,
+    },
+    {
+      kind: "assign",
+      localId: "bag",
+      value: { kind: "fieldGet", obj: self, className, field: PROPERTY_BAG, type: DYN, loc },
+      loc,
+    },
+  ];
+}
+
+/** Whether `value` reads local `id`. */
+function mentionsLocal(value: IrExpr, id: string): boolean {
+  return JSON.stringify(value).includes(`"localId":${JSON.stringify(id)}`);
+}
+
+/** `if (!hasOwn(bag, key)) defineProperty(bag, key, <undefined data field>)`:
+ * a field's creation in its instance's property bag. */
+function defineIfAbsent(lowerer: Lowerer, bag: IrExpr, key: IrExpr, loc: SrcLoc): IrStmt[] {
+  return [
+    {
+      kind: "if",
+      cond: {
+        kind: "unary",
+        op: "!",
+        operand: { kind: "libCall", fn: "dyn.hasOwnComputed", args: [bag, key], type: BOOL, loc },
+        type: BOOL,
         loc,
       },
-    ],
-    loc,
-  });
+      then: [
+        {
+          kind: "exprStmt",
+          expr: {
+            kind: "libCall",
+            fn: "dyn.defineProperty",
+            args: [
+              bag,
+              lowerer.coerceToExpected(key, DYN),
+              {
+                kind: "dynObjLit",
+                fields: [
+                  {
+                    key: { kind: "strLit", value: "value", type: STRING, loc },
+                    value: dynUndefinedExpr(loc),
+                  },
+                  ...["writable", "enumerable", "configurable"].map((key) => ({
+                    key: { kind: "strLit" as const, value: key, type: STRING, loc },
+                    value: lowerer.coerceToExpected(
+                      { kind: "boolLit", value: true, type: BOOL, loc },
+                      DYN,
+                    ),
+                  })),
+                ],
+                type: DYN,
+                loc,
+              },
+            ],
+            type: DYN,
+            loc,
+          },
+          loc,
+        },
+      ],
+      else_: null,
+      loc,
+    },
+  ];
 }
 
 /** Calls on native class capsules keep the instance's compiled methods. The
@@ -4648,6 +4843,7 @@ export class ClassDynamicDispatch {
   finalize(lowerer?: Lowerer): void {
     // Each table serves every site that reaches it.
     if (lowerer) for (const fn of this.generated) lowerer.shareHelper(fn);
+    if (lowerer) this.recordBagInitializers(lowerer);
     for (const { helper, initialize, orEmpty } of this.readPropertyBags) {
       const absent = helper.body[0];
       if (isEmptyBag(initialize.value) && !orEmpty) helper.body.shift();
@@ -4668,6 +4864,82 @@ export class ClassDynamicDispatch {
         fn.name === READ_PROPERTIES_OR_EMPTY,
       );
     }
+    if (lowerer) this.guardClassTables(lowerer);
+  }
+
+  /** Each tracked class's bag initializer over a typed receiver (`this`),
+   * for the per-class field-creation helpers (trackClassFieldCreation),
+   * which create an instance's bag without the class-table dispatch. */
+  private recordBagInitializers(lowerer: Lowerer): void {
+    for (const [className, initializers] of this.bagInitializers) {
+      const init = initializers[0];
+      if (!init || !lowerer.classes.get(className)?.def.tracksOwnFields) continue;
+      const self = varRef("this", { kind: "object", className }, init.loc);
+      const value = withTypedReceiver(structuredClone(init.value), self);
+      if (mentionsLocal(value, "p.0")) continue;
+      // Lowerer stand-ins in unit tests may not carry the table.
+      lowerer.classBagInitializers?.set(className, value);
+    }
+  }
+
+  /** Fast exits for the generated class tables: every class branch of
+   * the property-bag, normalization and static-bag helpers tests the
+   * value's capsule key (or class key), so a value of another kind (a
+   * plain object, the common case in untyped code) skips straight to the
+   * table's fallthrough instead of testing every class in the program. */
+  private guardClassTables(lowerer: Lowerer): void {
+    const names = new Set([
+      "%dyn.class.properties",
+      READ_PROPERTIES,
+      READ_PROPERTIES_OR_EMPTY,
+      "%dyn.class.normalize",
+      "%dyn.class.staticProperties",
+    ]);
+    for (const fn of lowerer.liftedFns) {
+      if (!names.has(fn.name) || fn.body.length < 2) continue;
+      const param = fn.params[0]?.localId;
+      if (param === undefined) continue;
+      const last = fn.body[fn.body.length - 1]!;
+      if (last.kind !== "return" || last.value === null || !returnsParam(last.value, param))
+        continue;
+      let test: "classCapsule" | "funcKind" | null = null;
+      let ok = true;
+      for (const stmt of fn.body.slice(0, -1)) {
+        if (
+          stmt.kind === "assign" &&
+          stmt.localId === param &&
+          stmt.value.kind === "call" &&
+          stmt.value.callee === "%dyn.class.normalize"
+        )
+          continue;
+        const kind =
+          stmt.kind === "if" && stmt.else_ === null ? branchKind(stmt.cond, param) : null;
+        if (kind === null || (test !== null && kind !== test)) {
+          ok = false;
+          break;
+        }
+        test = kind;
+      }
+      if (!ok || test === null) continue;
+      const first = fn.body[0]!;
+      if (first.kind === "if" && first.cond.kind === "dynTest" && first.cond.test === test)
+        continue;
+      const loc = fn.loc;
+      fn.body.unshift({
+        kind: "if",
+        cond: {
+          kind: "dynTest",
+          test,
+          negated: true,
+          value: varRef(param, DYN, loc),
+          type: BOOL,
+          loc,
+        },
+        then: [structuredClone(last)],
+        else_: null,
+        loc,
+      });
+    }
   }
 
   private normalizedDispatches(): IrFunction[] {
@@ -4741,10 +5013,17 @@ export class ClassDynamicDispatch {
       this.normalizationKey = key;
       const loc = this.normalization.loc;
       const value = varRef("value", DYN, loc);
-      this.normalization.body = entries.map(({ capsule, children }): IrStmt => {
+      // The checked receiver is unboxed once per capsule branch, not once
+      // per subclass test.
+      this.normalization.locals = this.normalization.locals.filter(
+        (local) => !local.id.startsWith("%normalized."),
+      );
+      this.normalization.body = entries.map(({ capsule, children }, index): IrStmt => {
         const type: IrType = { kind: "object", className: capsule.def.name };
-        const receiver: IrExpr = { kind: "dynCheck", value, type, loc };
-        return {
+        const id = `%normalized.${index}`;
+        this.normalization!.locals.push({ id, name: id, type, mutable: false });
+        const receiver: IrExpr = varRef(id, type, loc);
+        const branch: IrStmt = {
           kind: "if",
           cond: {
             kind: "libCall",
@@ -4785,6 +5064,15 @@ export class ClassDynamicDispatch {
           else_: null,
           loc,
         };
+        // The capsule test stays the branch condition; its body unboxes.
+        if (branch.kind === "if")
+          branch.then.unshift({
+            kind: "varDecl",
+            localId: id,
+            init: { kind: "dynCheck", value, type, loc },
+            loc,
+          });
+        return branch;
       });
       this.normalization.body.push({ kind: "return", value, loc });
     }
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index a765c91ee..66d3fd294 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -3646,6 +3646,9 @@ export class Lowerer {
       .some((sf) => isDeclBoundaryFile(sf.fileName));
     return this.declBoundaryMemo;
   }
+  /** Tracked classes' bag initializers over a typed `this`
+   * (class-dynamic-dispatch.ts recordBagInitializers). */
+  readonly classBagInitializers = new Map<string, IrExpr>();
 
   constructor(
     readonly program: ts.Program,
diff --git a/packages/compiler/src/ir/builtin-signatures.ts b/packages/compiler/src/ir/builtin-signatures.ts
index b7ecd57d2..82492c0f4 100644
--- a/packages/compiler/src/ir/builtin-signatures.ts
+++ b/packages/compiler/src/ir/builtin-signatures.ts
@@ -3745,6 +3745,10 @@ export const LIB_FN_SIGS = defineLibFnSignatures({
   "dyn.assignPrototype": { argTypes: [DYN, DYN, DYN], result: DYN },
   "dyn.definePrototypeProps": { argTypes: [DYN, DYN, DYN], result: DYN },
   "dyn.bagGet": { argTypes: [DYN, STRING, DYN], result: DYN },
+  /** A class field's creation in its instance's property bag: adds the
+   * string key as a default data property when absent, answering false
+   * (nothing changed) when the bag needs the general defineProperty path. */
+  "dyn.bagCreateField": { argTypes: [DYN, STRING], result: BOOL },
   "dyn.bagSet": { argTypes: [DYN, STRING, DYN, DYN], result: VOID },
   "dyn.objCreate": { argTypes: [DYN], result: DYN },
   // Names a compiled class's prototype object (args[0]) after the class
diff --git a/packages/compiler/src/ir/ir.ts b/packages/compiler/src/ir/ir.ts
index 63286ff54..6d0760b81 100644
--- a/packages/compiler/src/ir/ir.ts
+++ b/packages/compiler/src/ir/ir.ts
@@ -3445,7 +3445,11 @@ export type IrExpr =
         | "array"
         | "truthy"
         | "error"
-        | "function";
+        | "function"
+        /** The value is a capsule of a compiled class instance. */
+        | "classCapsule"
+        /** The value is a boxed function or class value (exact kind test). */
+        | "funcKind";
       bytesElem?: IrBytesElem;
       negated?: true;
       value: IrExpr;
diff --git a/packages/compiler/src/ir/js-ic.test.ts b/packages/compiler/src/ir/js-ic.test.ts
index 1b566fd00..f71a1e9db 100644
--- a/packages/compiler/src/ir/js-ic.test.ts
+++ b/packages/compiler/src/ir/js-ic.test.ts
@@ -176,3 +176,64 @@ test("a function holds a site when it calls a classified helper or dispatches a
   expect(hasJsIcSite({ ...invoker, name: "%init.0" }, sites)).toBe(false);
   expect(hasJsIcSite({ ...caller, name: "%init.0" }, sites)).toBe(true);
 });
+
+test("bag helpers that exit early for values no class branch matches are still the receiver", () => {
+  /** `if (!<test>(v)) return <value>`: the class tables' early exit. */
+  const exit = (test: "classCapsule" | "funcKind", value: IrExpr, negated = true): IrStmt => ({
+    kind: "if",
+    cond: {
+      kind: "dynTest",
+      test,
+      ...(negated ? { negated: true as const } : {}),
+      value: ref("v"),
+      type: BOOL,
+      loc,
+    },
+    then: [ret(value)],
+    else_: null,
+    loc,
+  });
+  const guarded = (name: string, first: IrStmt): IrFunction =>
+    helper(name, ["v"], [first, capsuleBranch("v"), ret(ref("v"))]);
+  const readBag = guarded("%dyn.class.readPropertiesOrEmpty", exit("classCapsule", ref("v")));
+  const writeBag = guarded("%dyn.class.properties", exit("classCapsule", ref("v")));
+  const staticBag = guarded("%dyn.class.staticProperties", exit("funcKind", ref("v")));
+  const getter = getHelper(
+    "%dyn.class.property.0",
+    "x",
+    [],
+    call(staticBag.name, call(readBag.name, ref("p.0"))),
+  );
+  const setter = setHelper("%dyn.class.property.1", "y");
+  // An exit that returns another value, or a test that is not negated,
+  // makes the bag differ from the receiver.
+  const otherExit = guarded(
+    "%dyn.class.other",
+    exit("classCapsule", { kind: "dynObjLit", type: DYN, loc } as IrExpr),
+  );
+  const positive = guarded("%dyn.class.positive", exit("classCapsule", ref("v"), false));
+  const otherGetter = getHelper("%dyn.class.property.2", "x", [], call(otherExit.name, ref("p.0")));
+  const positiveGetter = getHelper(
+    "%dyn.class.property.3",
+    "x",
+    [],
+    call(positive.name, ref("p.0")),
+  );
+  const sites = new JsIcSites({
+    functions: [
+      readBag,
+      writeBag,
+      staticBag,
+      otherExit,
+      positive,
+      getter,
+      setter,
+      otherGetter,
+      positiveGetter,
+    ],
+  });
+  expect(sites.site("%dyn.class.property.0")).toEqual({ kind: "get", key: "x" });
+  expect(sites.site("%dyn.class.property.1")).toEqual({ kind: "set", key: "y" });
+  expect(sites.site("%dyn.class.property.2")).toBeNull();
+  expect(sites.site("%dyn.class.property.3")).toBeNull();
+});
diff --git a/packages/compiler/src/ir/js-ic.ts b/packages/compiler/src/ir/js-ic.ts
index 316b3c958..752316db3 100644
--- a/packages/compiler/src/ir/js-ic.ts
+++ b/packages/compiler/src/ir/js-ic.ts
@@ -170,6 +170,18 @@ export class JsIcSites {
     );
   }
 
+  /** An early exit for values a class table never matches: `if (!<kind
+   * test>(value)) return <value>`, with no else. A plain object takes it
+   * and gets itself back. */
+  private kindExit(s: IrStmt, param: string): boolean {
+    if (s.kind !== "if" || s.else_ !== null || s.then.length !== 1) return false;
+    const { cond } = s;
+    if (cond.kind !== "dynTest" || cond.negated !== true || !isVar(cond.value, param)) return false;
+    if (cond.test !== "classCapsule" && cond.test !== "funcKind") return false;
+    const ret = s.then[0]!;
+    return ret.kind === "return" && ret.value !== null && this.bagOf(ret.value, param);
+  }
+
   /** A property-bag helper that returns its argument unless a capsule
    * branch is taken. */
   private identityHelper(name: string): boolean {
@@ -190,7 +202,9 @@ export class JsIcSites {
       const param = fn.params[0]!.localId;
       const last = fn.body[fn.body.length - 1];
       identity =
-        fn.body.slice(0, -1).every((s) => this.capsuleBranch(s, param)) &&
+        fn.body
+          .slice(0, -1)
+          .every((s) => this.capsuleBranch(s, param) || this.kindExit(s, param)) &&
         last?.kind === "return" &&
         last.value !== null &&
         this.bagOf(last.value, param);
diff --git a/packages/runtime/src/scr_json.c b/packages/runtime/src/scr_json.c
index 3cbebcd26..52fb53456 100644
--- a/packages/runtime/src/scr_json.c
+++ b/packages/runtime/src/scr_json.c
@@ -16140,3 +16140,27 @@ void scr_dyn_decl_become(ScrDyn *n, const ScrDyn *capsule) {
   }
   scr_dyn_release(old);
 }
+
+/* A compiled class's field creation in its instance's property bag (the
+ * compiler's trackClassFieldCreation): an ordinary extensible bag gains
+ * `key` as a writable, enumerable, configurable data property when it
+ * does not have it yet. Answers false, changing nothing, for any other bag,
+ * whose creation takes the general defineProperty path. Calls only
+ * exported functions. */
+bool scr_dyn_bag_create_field(ScrDyn *bag, const ScrStr *key);
+bool scr_dyn_bag_create_field(ScrDyn *bag, const ScrStr *key) {
+  if (!bag || bag->kind != SCR_DYN_OBJ || bag->non_extensible || bag->v.obj.source_access)
+    return false;
+  if (scr_dyn_obj_get(bag, key->data, key->len)) return true;
+  scr_dyn_obj_set(bag, key->data, key->len, scr_dyn_retain(scr_dyn_undefined()));
+  return true;
+}
+
+/* Whether a value is a capsule of a compiled class instance (its type key
+ * names an object type): the class tables' fast exit for every other value,
+ * including record and array capsules. Calls only libc. */
+bool scr_dyn_is_class_capsule(const ScrDyn *d);
+bool scr_dyn_is_class_capsule(const ScrDyn *d) {
+  return d && d->kind == SCR_DYN_TYPED_REF && d->v.typed_ref.type_key_len > 7 &&
+         memcmp(d->v.typed_ref.type_key, "object:", 7) == 0;
+}