Comment 2 for bug 1004251

This sounds like a regression of bug 930192. That bug is still fixed, but overall rendering performance in Ubuntu 12.04 has regressed still when using Unity. I believe this is a problem with the unityshell plugin because performance becomes fast again if I (briefly) disable Unity.

In the mean time, please try this potential workaround:
1. Run ccsm
2. Go to Workarounds
3. Enable "Force full screen redraws (buffer swap) on repaint"