[telepathy mode on]
I guess you use last Xorg of version 1.11.*. If it is then you should try to downgrade Xorg to something like 1.10.* (in Debian is 1.6.*).
You're the one of many experiencing the performance regression with last Xorg. There are a few threads here about similar problems.
It's all about trapezoid acceleration. There is the unofficial path for temporary solution.
Please, be patient and wait for next driver update.
Or downgrade Xorg as suggested.
[/telepathy mode off]