Comment 11 for bug 360713

cmavr - I've seen what you're describing. When the virtual desktop is larger than 2048, all video rendering was done in sw (compiz disabled) and yes, usability is a serious pain. Scrolling (and other 2d acceleration) works just fine w/windows.

Looks like there's a fresh thread that discusses this issue:
https://bugs.launchpad.net/ubuntu/+source/mesa/+bug/146859

Looks like the main bug that tracks this issue