> I have other patch that makes `a + b` yet tens percents faster, faster than `a.__add__(b)`, by adding functions and declarations that are never used. Confusing.

It seems like you are a victim of the "deadcode" issue related to code locality:
https://haypo.github.io/journey-to-stable-benchmark-deadcode.html

To run microbenchmarks on such very tiny functions taken less than 200 ns, it's more reliable to compile Python using LTO+PGO.