diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5892-buffer-write-utf16le-runs.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5892-buffer-write-utf16le-runs.ts.json
new file mode 100644
index 000000000..012edf8c1
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5892-buffer-write-utf16le-runs.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/5892-buffer-write-utf16le-runs.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/runtime/src/scr_bytes.c b/packages/runtime/src/scr_bytes.c
index e0fb55464..df5b8a3c0 100644
--- a/packages/runtime/src/scr_bytes.c
+++ b/packages/runtime/src/scr_bytes.c
@@ -17,6 +17,11 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#if defined(__SSE2__)
+#include <emmintrin.h>
+#elif defined(__ARM_NEON) && defined(__aarch64__)
+#include <arm_neon.h>
+#endif
 
 static bool scr_td_label_space(unsigned char c) {
   return c == 9 || c == 10 || c == 12 || c == 13 || c == 32;
@@ -3302,13 +3307,30 @@ double scr_bytes_write_str(ScrBytes *b, const ScrStr *s, const ScrStr *enc,
     while (i < s->len && budget - written >= width) {
 #if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
       /* utf16le of an ASCII run: zero-extend each byte to a little-endian
-       * unit, eight per step with two word stores (the generic loop below
-       * stores byte by byte, because the source and destination may alias
-       * as far as the compiler knows). */
+       * unit, sixteen per step with vector unpacks where the target has them
+       * and eight per step with two word stores otherwise (the generic loop
+       * below stores byte by byte, because the source and destination may
+       * alias as far as the compiler knows). */
       if (utf16) {
         uint8_t *d = b->data + o + written;
         size_t n = 0, room = (budget - written) / 16, left = (s->len - i) / 8;
         size_t steps = room < left ? room : left;
+#if defined(__SSE2__)
+        const __m128i zero = _mm_setzero_si128();
+        for (; n + 2 <= steps; n += 2) {
+          __m128i v = _mm_loadu_si128((const __m128i *)(s->data + i + 8 * n));
+          if (_mm_movemask_epi8(v)) break;
+          _mm_storeu_si128((__m128i *)(d + 16 * n), _mm_unpacklo_epi8(v, zero));
+          _mm_storeu_si128((__m128i *)(d + 16 * n + 16), _mm_unpackhi_epi8(v, zero));
+        }
+#elif defined(__ARM_NEON) && defined(__aarch64__)
+        for (; n + 2 <= steps; n += 2) {
+          uint8x16_t v = vld1q_u8((const uint8_t *)s->data + i + 8 * n);
+          if (vmaxvq_u8(v) & 0x80) break;
+          vst1q_u8(d + 16 * n, vreinterpretq_u8_u16(vmovl_u8(vget_low_u8(v))));
+          vst1q_u8(d + 16 * n + 16, vreinterpretq_u8_u16(vmovl_high_u8(v)));
+        }
+#endif
         for (; n < steps; n++) {
           uint64_t w;
           memcpy(&w, s->data + i + 8 * n, 8);
diff --git a/tests/corpus/5892-buffer-write-utf16le-runs.ts b/tests/corpus/5892-buffer-write-utf16le-runs.ts
new file mode 100644
index 000000000..a69c0e8d5
--- /dev/null
+++ b/tests/corpus/5892-buffer-write-utf16le-runs.ts
@@ -0,0 +1,60 @@
+// Buffer.write(text, "utf16le") over ASCII runs of every length around the
+// sixteen-byte widening steps (4732 covers the eight-byte ones): odd and even
+// offsets, length limits that cut a run, and non-ASCII, astral and
+// lone-surrogate units at every position of a 41-unit run.
+
+function hex(bytes: Uint8Array): string {
+  let out = "";
+  for (let i = 0; i < bytes.length; i++) out += (bytes[i]! < 16 ? "0" : "") + bytes[i]!.toString(16);
+  return out;
+}
+
+function digest(bytes: Uint8Array): number {
+  let h = 0x811c9dc5 | 0;
+  for (let i = 0; i < bytes.length; i++) h = Math.imul(h ^ bytes[i]!, 0x01000193);
+  return h >>> 0;
+}
+
+const alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_$";
+function run(n: number): string {
+  let s = "";
+  for (let i = 0; i < n; i++) s += alphabet[i % alphabet.length];
+  return s;
+}
+
+let summary = 0;
+for (let n = 0; n <= 70; n++) {
+  const text = run(n);
+  for (const offset of [0, 1, 3, 16]) {
+    const buf = Buffer.alloc(offset + 2 * n + 5, 0xee);
+    const written = buf.write(text, offset, "utf16le");
+    summary = (Math.imul(summary, 31) + written + digest(buf)) | 0;
+    for (const limit of [0, 1, 15, 16, 17, 31, 32, 33]) {
+      if (limit > 2 * n + 5) continue;
+      const cut = Buffer.alloc(offset + 2 * n + 5, 0xee);
+      const w = cut.write(text, offset, limit, "utf16le");
+      summary = (Math.imul(summary, 31) + w + digest(cut)) | 0;
+    }
+  }
+}
+console.log("runs", summary);
+
+for (const insert of ["é", "€", "😀", "\ud800", "\udc00", "\u0080", "ÿ"]) {
+  let local = 0;
+  for (let at = 0; at <= 40; at++) {
+    const text = run(at) + insert + run(40 - at);
+    const buf = Buffer.alloc(2 * text.length + 3, 0xee);
+    const written = buf.write(text, 1, "utf16le");
+    local = (Math.imul(local, 31) + written + digest(buf)) | 0;
+    if (at === 17) console.log(JSON.stringify(insert), written, hex(buf.subarray(33, 41)));
+  }
+  console.log("insert", JSON.stringify(insert), local);
+}
+
+// A code-unit array the way a scanner builds one, then read back.
+const source = run(1000) + "\n" + "const x = 1;\r\n".repeat(50);
+const units = new Uint16Array(source.length + 4);
+const n = Buffer.from(units.buffer).write(source, "utf16le");
+let same = n === 2 * source.length;
+for (let i = 0; i < source.length; i++) if (units[i] !== source.charCodeAt(i)) same = false;
+console.log("units", n, same, units[source.length], units[source.length + 3]);