[CentOS] Clunky Xorg with latest 6.7 upgrade?

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.