> Yes, that slows down Python 3.7.0a3 to the 3.7.0a4 level.

Ok, so something calls setlocale(LC_CTYPE, "") or setlocale(LC_ALL, "") in Python 3.7.0. I'm not interested to dig the Git history. It doesn't really matter at this point.

Can you try to get the current LC_CTYPE locale on Python 3.6 or 3.7.0a3? Example:

$ python3 -c 'import _locale; print(_locale.setlocale(_locale.LC_CTYPE, None))'
fr_FR.utf8

Or using the locale module (it should give the same result):

$ python3 -c 'import locale; print(locale.setlocale(locale.LC_CTYPE, None))'
fr_FR.utf8

Can also also try on 3.7.0a4 and newer (ex: Python 3.7.1): locale.setlocale(locale.LC_ALL, "C"), to see it does workaround your performance issue? I don't recall if "C" or "POSIX" locales are supported on Windows.