Guido van Rossum <gu...@python.org> added the comment:

I'm still wondering why speed.python.org showed such a slowdown, and how we can 
revert that. Here's a new theory.

Thanks to an investigation I did together with Eric, I now suspect that the 
release of setuptools 60.0.0 on Dec 19 is a smoking gun. PyPerformance 
(re)installs the latest versions of pip and setuptools.

Setuptools 60.0 makes a change in the file distutils-precedence.pth that causes 
it (by default) to import something called _distutils_hack and to call its 
add_shim() function. In previous setuptools this was off by default, but in 
60.0 it switched to on. See 
https://github.com/pypa/setuptools/blob/main/CHANGES.rst#v6000

That add_shim() call in turn installs an extra entry in front of sys.meta_path, 
which does a bunch of extra work. See 
https://github.com/pypa/setuptools/blob/main/_distutils_hack/__init__.py

Pablo, can we change the PyPerformance configuration or the script that runs it 
to set and export SETUPTOOLS_USE_DISTUTILS=stdlib, to see whether that affects 
perf at all?

(Note that the code in distutils-precedence.pth is executed by site.py in 
addpackage().)

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