> Can someone please try to write an example which only uses the stdlib?

The simplest is to compare performance of the `windll.LoadLibrary('API-MS-WIN-CRT-STRING-L1-1-0.DLL')` function on Python 3.7.0a3 and 3.7.0a4, but that will mostly measure Python/ctypes overhead. I will post a minimal C extension instead.


> What are these extensions? Where do them come from?

The `isdigit` function is from the UCRT. The `parsers` Cython/C extension is part of the pandas wheel on PyPI. The context for this issue is at https://github.com/pandas-dev/pandas/issues/23516


> I don't understand which "locale changes" you are talking about. You can change the locale using locale.setlocale().

The `UCRT.isdigit` function, when run on Python >=3.7.0a4, calls the `_isdigit_l` function, which calls `_LocaleUpdate::_LocaleUpdate` (see the VS profiler output).