This is something I saw somewhere else but cannot remember where.
If you disable all of the ATI extensions in Classic (which aren't needed if you're running OS X) then you seem to get better performance in some Classic applications.
[Editor's note: As an expansion of this theory, there's a bunch of stuff that I've disabled in Classic - nearly everything, in fact. I basically installed just the base OS with networking, and nothing else. Then I installed (running the "Classic" 9.1 natively) all my apps that put bits into the system folder (Office98, goLive, etc.). Then I restarted into X and set my lean OS 9.1 as my Classic volume. You can definitely improve performance and decrease loading time by thinning your Classic system -- which is another argument for having your "real" OS 9.1 on another partition, so you don't have to mess around with extension sets.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010516012428192