Just upgraded to 12.04 and my video performance feels a lot worse. Running an Intel GM45 with dual 1280x1900 screens. When I switch to a single screen performance is much better. This was the same setup on my 11.10 system, and was fine with the dual screens.
glxgears is running around 30fps with dual screens on, and 60fps (vsync limted?) with a single screen.
Looking for some ideas, or anyone else with the same issues.
Have you tried to change you desktop manager ? Try gnome shell or xfce to see if it related with this incredibly feature rich and heavy consuming new desktop experience ?
Another idea would be to go with the 2D desktop to see if you got better performance. Or disabled extra visual effects.