@@ -63,10 +63,12 @@ PyAPI_FUNC(Py_complex) PyComplex_AsCComplex(PyObject *op);
 /* Format the object based on the format_spec, as defined in PEP 3101
    (Advanced String Formatting). */
 #ifndef Py_LIMITED_API
-PyAPI_FUNC(PyObject *) _PyComplex_FormatAdvanced(PyObject *obj,
-                                                 PyObject *format_spec,
-                                                 Py_ssize_t start,
-                                                 Py_ssize_t end);
+PyAPI_FUNC(int) _PyComplex_FormatAdvancedWriter(
+    _PyUnicodeWriter *writer,
+    PyObject *obj,
+    PyObject *format_spec,
+    Py_ssize_t start,
+    Py_ssize_t end);
 #endif
 
 #ifdef __cplusplus
 
@@ -112,10 +112,12 @@ PyAPI_FUNC(int) PyFloat_ClearFreeList(void);
 
 /* Format the object based on the format_spec, as defined in PEP 3101
    (Advanced String Formatting). */
-PyAPI_FUNC(PyObject *) _PyFloat_FormatAdvanced(PyObject *obj,
-                                               PyObject *format_spec,
-                                               Py_ssize_t start,
-                                               Py_ssize_t end);
+PyAPI_FUNC(int) _PyFloat_FormatAdvancedWriter(
+    _PyUnicodeWriter *writer,
+    PyObject *obj,
+    PyObject *format_spec,
+    Py_ssize_t start,
+    Py_ssize_t end);
 #endif /* Py_LIMITED_API */
 
 #ifdef __cplusplus
 
@@ -151,14 +151,22 @@ PyAPI_FUNC(int) _PyLong_AsByteArray(PyLongObject* v,
 
 /* _PyLong_Format: Convert the long to a string object with given base,
    appending a base prefix of 0[box] if base is 2, 8 or 16. */
-PyAPI_FUNC(PyObject *) _PyLong_Format(PyObject *aa, int base);
+PyAPI_FUNC(PyObject *) _PyLong_Format(PyObject *obj, int base);
+
+PyAPI_FUNC(int) _PyLong_FormatWriter(
+    _PyUnicodeWriter *writer,
+    PyObject *obj,
+    int base,
+    int alternate);
 
 /* Format the object based on the format_spec, as defined in PEP 3101
    (Advanced String Formatting). */
-PyAPI_FUNC(PyObject *) _PyLong_FormatAdvanced(PyObject *obj,
-                                              PyObject *format_spec,
-                                              Py_ssize_t start,
-                                              Py_ssize_t end);
+PyAPI_FUNC(int) _PyLong_FormatAdvancedWriter(
+    _PyUnicodeWriter *writer,
+    PyObject *obj,
+    PyObject *format_spec,
+    Py_ssize_t start,
+    Py_ssize_t end);
 #endif /* Py_LIMITED_API */
 
 /* These aren't really part of the long object, but they're handy. The
 
@@ -648,8 +648,20 @@ PyAPI_FUNC(Py_ssize_t) PyUnicode_CopyCharacters(
     Py_ssize_t from_start,
     Py_ssize_t how_many
     );
+
+/* Unsafe version of PyUnicode_CopyCharacters(): don't check arguments and so
+   may crash if parameters are invalid (e.g. if the output string
+   is too short). */
+PyAPI_FUNC(void) _PyUnicode_FastCopyCharacters(
+    PyObject *to,
+    Py_ssize_t to_start,
+    PyObject *from,
+    Py_ssize_t from_start,
+    Py_ssize_t how_many
+    );
 #endif
 
+#ifndef Py_LIMITED_API
 /* Fill a string with a character: write fill_char into
    unicode[start:start+length].
 
@@ -658,13 +670,21 @@ PyAPI_FUNC(Py_ssize_t) PyUnicode_CopyCharacters(
 
    Return the number of written character, or return -1 and raise an exception
    on error. */
-#ifndef Py_LIMITED_API
 PyAPI_FUNC(Py_ssize_t) PyUnicode_Fill(
     PyObject *unicode,
     Py_ssize_t start,
     Py_ssize_t length,
     Py_UCS4 fill_char
     );
+
+/* Unsafe version of PyUnicode_Fill(): don't check arguments and so may crash
+   if parameters are invalid (e.g. if length is longer than the string). */
+PyAPI_FUNC(void) _PyUnicode_FastFill(
+    PyObject *unicode,
+    Py_ssize_t start,
+    Py_ssize_t length,
+    Py_UCS4 fill_char
+    );
 #endif
 
 /* Create a Unicode Object from the Py_UNICODE buffer u of the given
@@ -696,13 +716,19 @@ PyAPI_FUNC(PyObject*) PyUnicode_FromString(
     const char *u              /* UTF-8 encoded string */
     );
 
+#ifndef Py_LIMITED_API
 /* Create a new string from a buffer of Py_UCS1, Py_UCS2 or Py_UCS4 characters.
    Scan the string to find the maximum character. */
-#ifndef Py_LIMITED_API
 PyAPI_FUNC(PyObject*) PyUnicode_FromKindAndData(
     int kind,
     const void *buffer,
     Py_ssize_t size);
+
+/* Create a new string from a buffer of ASCII characters.
+   WARNING: Don't check if the string contains any non-ASCII character. */
+PyAPI_FUNC(PyObject*) _PyUnicode_FromASCII(
+    const char *buffer,
+    Py_ssize_t size);
 #endif
 
 PyAPI_FUNC(PyObject*) PyUnicode_Substring(
@@ -864,13 +890,70 @@ PyAPI_FUNC(PyObject *) PyUnicode_FromFormat(
     ...
     );
 
+#ifndef Py_LIMITED_API
+typedef struct {
+    PyObject *buffer;
+    void *data;
+    enum PyUnicode_Kind kind;
+    Py_UCS4 maxchar;
+    Py_ssize_t size;
+    Py_ssize_t pos;
+    /* minimum length of the buffer when overallocation is enabled,
+       see _PyUnicodeWriter_Init() */
+    Py_ssize_t min_length;
+    struct {
+        unsigned char overallocate:1;
+        /* If readonly is 1, buffer is a shared string (cannot be modified)
+           and size is set to 0. */
+        unsigned char readonly:1;
+    } flags;
+} _PyUnicodeWriter ;
+
+/* Initialize a Unicode writer.
+
+   If min_length is greater than zero, _PyUnicodeWriter_Prepare()
+   overallocates the buffer and min_length is the minimum length in characters
+   of the buffer. */
+PyAPI_FUNC(void)
+_PyUnicodeWriter_Init(_PyUnicodeWriter *writer, Py_ssize_t min_length);
+
+/* Prepare the buffer to write 'length' characters
+   with the specified maximum character.
+
+   Return 0 on success, raise an exception and return -1 on error. */
+#define _PyUnicodeWriter_Prepare(WRITER, LENGTH, MAXCHAR)             \
+    (((MAXCHAR) <= (WRITER)->maxchar                                  \
+      && (LENGTH) <= (WRITER)->size - (WRITER)->pos)                  \
+     ? 0                                                              \
+     : (((LENGTH) == 0)                                               \
+        ? 0                                                           \
+        : _PyUnicodeWriter_PrepareInternal((WRITER), (LENGTH), (MAXCHAR))))
+
+/* Don't call this function directly, use the _PyUnicodeWriter_Prepare() macro
+   instead. */
+PyAPI_FUNC(int)
+_PyUnicodeWriter_PrepareInternal(_PyUnicodeWriter *writer,
+                                 Py_ssize_t length, Py_UCS4 maxchar);
+
+PyAPI_FUNC(int)
+_PyUnicodeWriter_WriteStr(_PyUnicodeWriter *writer, PyObject *str);
+
+PyAPI_FUNC(PyObject *)
+_PyUnicodeWriter_Finish(_PyUnicodeWriter *writer);
+
+PyAPI_FUNC(void)
+_PyUnicodeWriter_Dealloc(_PyUnicodeWriter *writer);
+#endif
+
 #ifndef Py_LIMITED_API
 /* Format the object based on the format_spec, as defined in PEP 3101
    (Advanced String Formatting). */
-PyAPI_FUNC(PyObject *) _PyUnicode_FormatAdvanced(PyObject *obj,
-                                                 PyObject *format_spec,
-                                                 Py_ssize_t start,
-                                                 Py_ssize_t end);
+PyAPI_FUNC(int) _PyUnicode_FormatAdvancedWriter(
+    _PyUnicodeWriter *writer,
+    PyObject *obj,
+    PyObject *format_spec,
+    Py_ssize_t start,
+    Py_ssize_t end);
 #endif
 
 PyAPI_FUNC(void) PyUnicode_InternInPlace(PyObject **);
 
@@ -13,6 +13,9 @@ Core and Builtins
 - Issue #14835: Make plistlib output empty arrays & dicts like OS X.
   Patch by Sidney San Martín.
 
+- Issue #14744: Use the new _PyUnicodeWriter internal API to speed up
+  str%args and str.format(args).
+
 - Issue #14930: Make memoryview objects weakrefable.
 
 - Issue #14775: Fix a potential quadratic dict build-up due to the garbage
 
@@ -699,11 +699,22 @@ static PyObject *
 complex__format__(PyObject* self, PyObject* args)
 {
     PyObject *format_spec;
+    _PyUnicodeWriter writer;
+    int ret;
 
     if (!PyArg_ParseTuple(args, "U:__format__", &format_spec))
-    return NULL;
-    return _PyComplex_FormatAdvanced(self, format_spec, 0,
-                                     PyUnicode_GET_LENGTH(format_spec));
+        return NULL;
+
+    _PyUnicodeWriter_Init(&writer, 0);
+    ret = _PyComplex_FormatAdvancedWriter(
+        &writer,
+        self,
+        format_spec, 0, PyUnicode_GET_LENGTH(format_spec));
+    if (ret == -1) {
+        _PyUnicodeWriter_Dealloc(&writer);
+        return NULL;
+    }
+    return _PyUnicodeWriter_Finish(&writer);
 }
 
 #if 0
 
@@ -267,13 +267,15 @@ static PyObject *
 float_repr(PyFloatObject *v)
 {
     PyObject *result;
-    char *buf = PyOS_double_to_string(PyFloat_AS_DOUBLE(v),
-                                      'r', 0,
-                                      Py_DTSF_ADD_DOT_0,
-                                      NULL);
+    char *buf;
+
+    buf = PyOS_double_to_string(PyFloat_AS_DOUBLE(v),
+                                'r', 0,
+                                Py_DTSF_ADD_DOT_0,
+                                NULL);
     if (!buf)
         return PyErr_NoMemory();
-    result = PyUnicode_FromString(buf);
+    result = _PyUnicode_FromASCII(buf, strlen(buf));
     PyMem_Free(buf);
     return result;
 }
@@ -1703,11 +1705,22 @@ static PyObject *
 float__format__(PyObject *self, PyObject *args)
 {
     PyObject *format_spec;
+    _PyUnicodeWriter writer;
+    int ret;
 
     if (!PyArg_ParseTuple(args, "U:__format__", &format_spec))
         return NULL;
-    return _PyFloat_FormatAdvanced(self, format_spec, 0,
-                                   PyUnicode_GET_LENGTH(format_spec));
+
+    _PyUnicodeWriter_Init(&writer, 0);
+    ret = _PyFloat_FormatAdvancedWriter(
+        &writer,
+        self,
+        format_spec, 0, PyUnicode_GET_LENGTH(format_spec));
+    if (ret == -1) {
+        _PyUnicodeWriter_Dealloc(&writer);
+        return NULL;
+    }
+    return _PyUnicodeWriter_Finish(&writer);
 }
 
 PyDoc_STRVAR(float__format__doc,