Mean +- std dev: [main] 9.25 ms +- 0.07 ms -> [patched] 7.68 ms +- 0.03 ms: 1.20x faster

$ ./python -m pyperf compare_to with-fast-path.json use_write_ascii.json -G
Slower (3):
- json_dumps(ensure_ascii=False): List of 256 dicts with 1 int: 101 us +- 0 us -> 102 us +- 0 us: 1.00x slower
- json_loads: Dict with 256 lists of 256 dicts with 1 int: 46.6 ms +- 0.1 ms -> 46.8 ms +- 0.5 ms: 1.00x slower
- json_dumps(ensure_ascii=False): List of 256 floats: 239 us +- 1 us -> 239 us +- 1 us: 1.00x slower

Faster (10):
- json_dumps(ensure_ascii=False): List of 256 strings: 303 us +- 5 us -> 279 us +- 3 us: 1.08x faster
- json_dumps: List of 256 strings: 302 us +- 3 us -> 278 us +- 3 us: 1.08x faster
- json_dumps(ensure_ascii=False): List of 256 booleans: 16.5 us +- 0.1 us -> 15.3 us +- 0.1 us: 1.08x faster
- json_dumps: List of 256 booleans: 16.5 us +- 0.1 us -> 15.3 us +- 0.1 us: 1.07x faster
- json_dumps: Complex object: 1.96 ms +- 0.01 ms -> 1.87 ms +- 0.01 ms: 1.05x faster
- json_dumps(ensure_ascii=False): Complex object: 1.96 ms +- 0.01 ms -> 1.87 ms +- 0.02 ms: 1.05x faster
- json_dumps: Medium complex object: 173 us +- 1 us -> 171 us +- 1 us: 1.01x faster
- json_dumps(ensure_ascii=False): Medium complex object: 172 us +- 1 us -> 171 us +- 1 us: 1.01x faster
- json_loads: Medium complex object: 148 us +- 1 us -> 147 us +- 1 us: 1.00x faster
- json_dumps: List of 256 floats: 239 us +- 0 us -> 239 us +- 0 us: 1.00x faster

Benchmark hidden because not significant (13): json_dumps: List of 256 ASCII strings, json_dumps: List of 256 dicts with 1 int, json_dumps: Dict with 256 lists of 256 dicts with 1 int, json_dumps(ensure_ascii=False): List of 256 ASCII st
rings, json_dumps(ensure_ascii=False): Dict with 256 lists of 256 dicts with 1 int, json_loads: List of 256 booleans, json_loads: List of 256 ASCII strings, json_loads: List of 256 floats, json_loads: List of 256 dicts with 1 int, json_l
oads: List of 256 strings, json_loads: Complex object, json_loads: List of 256 stringsensure_ascii=False, json_loads: Complex objectensure_ascii=False

diff --git a/Modules/_json.c b/Modules/_json.c
index cd08fa688d3..cd57760282a 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -351,7 +351,7 @@ write_escaped_ascii(PyUnicodeWriter *writer, PyObject *pystr)
         }

         if (buf_len + 12 > ESCAPE_BUF_SIZE) {
-            ret = PyUnicodeWriter_WriteUTF8(writer, buf, buf_len);
+            ret = _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, buf, buf_len);
             if (ret) return ret;
             buf_len = 0;
         }
@@ -359,7 +359,7 @@ write_escaped_ascii(PyUnicodeWriter *writer, PyObject *pystr)

     assert(buf_len < ESCAPE_BUF_SIZE);
     buf[buf_len++] = '"';
-    return PyUnicodeWriter_WriteUTF8(writer, buf, buf_len);
+    return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, buf, buf_len);
 }

 static int
@@ -1612,13 +1612,13 @@ encoder_listencode_obj(PyEncoderObject *s, PyUnicodeWriter *writer,
     int rv;

     if (obj == Py_None) {
-      return PyUnicodeWriter_WriteUTF8(writer, "null", 4);
+      return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, "null", 4);
     }
     else if (obj == Py_True) {
-      return PyUnicodeWriter_WriteUTF8(writer, "true", 4);
+      return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, "true", 4);
     }
     else if (obj == Py_False) {
-      return PyUnicodeWriter_WriteUTF8(writer, "false", 5);
+      return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, "false", 5);
     }
     else if (PyUnicode_Check(obj)) {
         return encoder_write_string(s, writer, obj);
@@ -1779,7 +1779,7 @@ encoder_listencode_dict(PyEncoderObject *s, PyUnicodeWriter *writer,

     if (PyDict_GET_SIZE(dct) == 0) {
         /* Fast path */
-        return PyUnicodeWriter_WriteUTF8(writer, "{}", 2);
+        return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, "{}", 2);
     }

     if (s->markers != Py_None) {
@@ -1883,7 +1883,7 @@ encoder_listencode_list(PyEncoderObject *s, PyUnicodeWriter *writer,
         return -1;
     if (PySequence_Fast_GET_SIZE(s_fast) == 0) {
         Py_DECREF(s_fast);
-        return PyUnicodeWriter_WriteUTF8(writer, "[]", 2);
+        return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, "[]", 2);
     }

     if (s->markers != Py_None) {