Function isspace() is significantly slower in extension modules in Python 3.7 or later than in Python 3.6 on Windows 10.

Python 3.6:
>python test.py
Locale: C
Set locale: C
Locale: C
Duration: 0.19718074798583984

Python 3.7:
>python test.py
Locale: C
Set locale: C
Locale: C
Duration: 2.9086477756500244

Python 3.8:
>python test.py
Locale: LC_COLLATE=C;LC_CTYPE=English_Germany.1252;LC_MONETARY=C;LC_NUMERIC=C;LC_TIME=C
Set locale: C
Locale: C
Duration: 2.99224853515625

Python 3.9.0a5:
>python test.py
Locale: LC_COLLATE=C;LC_CTYPE=English_Germany.1252;LC_MONETARY=C;LC_NUMERIC=C;LC_TIME=C
Set locale: C
Locale: C
Duration: 2.90370512008667

Building the extension module:
> python setup.py build_ext

A similar issue already exists, but I am not sure if it is the same problem since the suggested solution is about embedding Python rather then extending it: https://bugs.python.org/issue35195