My understanding is that compiler flags yielding speed boosts where an actual thing back in the day, but not anymore really.

Nowadays the speed come from other things. For example a few years ago i realised my firefox on linux was doing video encoding/decoding on cpu without using the intel gpu’s hardware encoder/decoder. I did not have to rebuild firefox from scratch but rather fiddle with configurations and environment variables to make sure Firefox wouldn’t assume the lowest common denominator.

That saved battery on my laptop and yielded better performance.