I suppose that my computer is a bit faster than your, so your 20 ns can be only 15 ns or 10 ns on my computer. Run microbenchmarks on your computer to get a scale.

It may be possible to save yet few nanoseconds if inline a fast path for _PyArg_CheckPositional(), but I'm going to try this later.

This change is a step in a sequence. I will add a What's New note after finishing so much steps as possible. The next large step is to optimize argument parsing for functions with keyword parameters.