I trust that you've done things like turn off all the services that you aren't using (Apache, Samba, etc.), and disabled things like dock effects.
Aqua will continue to be optimized, and improve. XP & Win2k are very old codebases (NT based), so have had much more time to be optimized. OSX is some old stuff (NeXT, BSD) merged with a lot of very new stuff (QE, Aqua, Cocoa, Carbon, etc.), and so much of the optimization has been sacrificed for stability's sake.
Remember how unbearably slow 10.0.0 was? Compare it to Jaguar on the same machine, and you'll see the effect I'm talking about.