It's a rough port that suffers from some major redraw slowdowns, and it doesn't yet support many features like CD burning and full-screen visualizations. Nevertheless, left minimized in a corner, it dutifully plays nearly skip-free MP3s. I don't doubt the potential of such Mach features, but I have not seen an application that takes full advantage of these abilities. Performance Summary Mac OS X is slower than Mac OS 9 on the same hardware. The interface is less responsive overall.
All classic applications take a minor speed hit.
The most comparable experience in the Mac world that I can recall is the introduction of color. 24-bit color display card. Likewise, the introduction of System 7 brought with it a huge performance hit. I could literally watch as the OS drew the menu, the menu items, and all the icons next to them. System 6 on the same hardware using the black and white display is considerably snappier.