> Isn't 800 lines of C code too high price for speeding up ABCs creation?

800 lines of C code is not something hard to notice, so I suppose the answer is obvious for all people involved in the work on PR :-)

> ...this can save just several milliseconds at start-up.

The correct way to measure this is relative, not absolute. There are just few ABCs used by modules loaded at Python start-up, and it already allowed to save 10% of start-up time. My expectation is that the number will be similar for a typical Python app. Moreover, `isinstance` and `issubclass` (functions called often with ABCs) will be 1.5x faster.