I can see how the compiler and stdlib (aren't things like memcpy implemented in hand-tuned assembly?) could be highly relevant, though.

They are, but that’s really low hanging fruit. If Android for example doesn’t use optimised memcpy implementations then they don’t deserve to exist as a serious OS.

That reminds me, when I read fast code in the 90s-2000s all the asm hackers were into writing their own cool memcpy. Were they just showing off, or did Windows actually never optimize their standard library?

People still seem to like writing their own cool malloc, but memcpy not so much.

There are some cases where it may make sense to write your own implementation, if you have a niche microarchitecture that has unusual performance characteristics that the OS doesn't provide optimized routines for by default. But for most u-archs the default optimized routines should do a good job.

Things like malloc are quite a bit more complicated, and more workload dependent so there's still some opportunity specializing an implementation there.