But if you look at your program that must go faster, and you see unnecessary moves in the hot section(s), go ahead and remove them, but don't be surprised if it doesn't change much.

If you went and did your whole program by hand, the debloating might also not change much. That's why there's a rule of thumb.

If you have the skill to make a change to the compiler so it can output a better sequence of instructions, I suspect thsat's pretty difficult, but it may make enough of a difference over a large number of programs to be worthwhile.