Robohornet's reflow test spends 2% of total time in ScrollbarThemeMac::registerScrollbar calling NSClassFromString("ScrollBarImp")
An easy fix would be to cache the class pointer, since it's unlikely to change. :)
Unclear how much (if at all) this affects real web content.
Actually, I don't think it's the ClassFromString lookup. This may just be a dupe of bug 98622.
*** This bug has been marked as a duplicate of bug 98622 ***