Vladimir Vukićević from Mozilla's Firefox team eventually managed to turn Firefox 3 into a speed demon on Mac OS X. But Apple sure didn't help with the process.
Apple may not have been trying to cripple non-Apple applications on Mac OS X, but the fact that it's not open source means that the world is beholden to Apple's whims, as Vlad writes:
I do not think that Apple is in any way trying to purposely "cripple" non-Apple software. I also do not think that undocumented APIs give Safari any kind of "significant performance advantage" (as Firefox 3 should show!). … Read more