X performance become really low after recovering from black screen

Reproducing:
Enable screen power saving, after the idle time and screen goes black, click the mouse or keyboard to make screen recovering, and enter the password to goback to desktop, and you will find window or unity animation is really slow.

The screen goes black I mean screen power off automaticly, like xset DPMS off. (I am not sure xset dpms off will cause the same problem, i will check it later.)

ubuntu 12.10, using ati open source xorg driver.
Acer V3 551G laptop:
AMD A8-4500M
DDR3 1333 4G ( x2 )
HD7640G+HD7670M

BTW: it seems it's not caused by gnome-power-manager, because when I lock the screen, and the screen goes black but back light is still on, then go back to desktop, everything is still fine.

It's also fine if i suspend the system then recover.

My english is not good, hope you under stand what i am saying. ; )