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!).
However, as I said, the undocumented functionality could be useful for Firefox and other apps to implement things in an simpler (and potentially more efficient) manner. I don't think this is malicious, it's just an unfortunate cutting of corners that is way too easy for a company that's not fully open to do.
Apple benefits the better that all applications run on Mac OS X, not just Apple-developed applications. Throttling performance - wittingly or unwittingly - is not in its interest. Open source would resolve the issue in Apple's favor.