Where do you get that 90/10 split from? Just curious.
I forgot exactly where I got that number, but it's been a pretty good metric so far.
In a nutshell; the compiler is great a micro-optimizations and absolutely terrible at macro-optimizations. The former will get you a few percent of perf boosts while the later usually results in orders of magnitudes of performance gains.
Its near impossible to apply macro-optimizations at the end of a project without massive refactors.