I am experiencing this with 9.04 - upgraded last Friday and now my laptop takes 15 seconds to repaint the desktop coming from the screensaver (which takes 15 seconds after a key-press to show the password dialog). I keep getting the "app is not responding - Wait/Force Quit?" dialog and my load average is insanely high. Damn. Even tried reverting to the 2.4 driver, no cigar.
I installed Jaunty Alpha 4 the other day, and with the latest packages (plus libsdl from a PPA to fix a bug with OpenGL), the 3D performance on my X4500 has improved massively over 8.10. Tremulous has gone from ~13 fps to 56.6 fps!
Finally got around to benching the final version of 9.04 (upgraded from 8.10, not a clean install. No special settings in xorg.conf), and it averaged 76.9 fps this time! Quite an improvement, I'd say. Games through Wine and my closed-source Linux games still don't work, but I'm glad to see the invasive driver work is starting to pay off.