$ ./python -m timeit "format('abc')"
Unpatched:  5000000 loops, best of 5: 65 nsec per loop
Patched:    5000000 loops, best of 5: 42.4 nsec per loop

-23 ns on 65 ns: this is very significant! I spent like 6 months to implement "FASTCALL" to avoid a single tuple to pass positional arguments and it was only 20 ns faster per call. Additional 23 ns make the code way faster compared than Python without FASTCALL! I estimate something like 80 ns => 42 ns: 2x faster!

$ ./python -m timeit "'abc'.replace('x', 'y')"
Unpatched:  5000000 loops, best of 5: 101 nsec per loop
Patched:    5000000 loops, best of 5: 63.8 nsec per loop

-38 ns on 101 ns: that's even more significant! Wow, that's very impressive!

Please merge your PR, I want it now :-D

Can you maybe add a vague sentence in the Optimizations section of What's New in Python 3.8 ? Something like: "Parsing positional arguments in builtin functions has been made more efficient."? I'm not sure if "builtin" is the proper term here. Functions using Argument Clinic to parse their arguments?