What video card are you using? Xorg driver or proprietary binary thing? The next thing I'd do is boot the last kernel under which you saw "normal" performance. If booting the older kernel doesn't fix the problem, then the problem is probably the X11 server. At that point, I'd start using "yum downgrade" to revert individual components, and reboot until you find the change that fixes performance. xorg-x11-server-Xorg and xorg-x11-drv-<something> are the ones I'd check first.