There appear to still be serious issues with both the proprietary Nvidia and ATI drivers with Xorg 1.9 - kind of a shame. If you have an older Nvidia card, or an ATI card plus certain monitors, you're not going to be able to use the good drivers with 10.10 for awhile.
i've run into this issue after upgrading--cpu usage is high and graphics rendering is slow. after disabling the proprietary nvidia driver everything went back to normal.