> I don't understand how _PyUnicodeWriter could be slow. It does not overallocate by default. It's just wrapper to implement efficient memory management.

I misunderstood _PyUnicodeWriter.  I thought it caused one more allocation, but it doesn't.

But _PyUnicodeWriter is still slow, because gcc and clang are not smart enough to optimize _PyUnicodeWriter_Init() & _PyUnicodeWriter_Prepare().

See this example:

```
#define PY_SSIZE_T_CLEAN
#include <Python.h>

#define S(s) (s),strlen(s)

int
main(int argc, char *argv[])
{
    Py_Initialize();

    for (int i=0; i<100000000; i++) {
        //PyObject *s = PyUnicode_FromString("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
        PyObject *s = _PyUnicode_FromASCII(S("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"));
        Py_DECREF(s);
    }
    return 0;
}
```

PyUnicode_FromString() takes about 4 sec on my machine.  _PyUnicode_FromASCII() is about 2 sec.
By skipping _PyUnicodeWriter for ASCII string (GH-14283), PyUnicode_FromString() takes about 3 sec.

```
$ time ./x  # PyUnicode_FromString

real    0m4.085s
user    0m4.081s
sys     0m0.004s

$ time ./y  # PyUnicode_FromString (skip _PyUnicode_Writer, GH-14283)

real    0m2.988s
user    0m2.988s
sys     0m0.000s

$ time ./z  # _PyUnicode_FromASCII
$ time ./z

real    0m1.975s
user    0m1.975s
sys     0m0.000s
```