> Jeff once simultaneously reduced all binary sizes by 3% and raised the severity of a previously known low-priority Python bug to critical-priority in a single change that contained no Python code.

This sounds really plausible. A change to the C toolchain/library (for example, specialized/inlined memcpy) may affect binary sizes significantly, and may change the behavior of something the C standard leaves undefined (for example, memcpy with overlapping arguments).

I have such a Python bug right now because of something that fork()s in a way that can't posix_spawn(). One of those is a lot easier to make performant than the other.