Me too. I was using an ATI card, without any 3d accel. It worked fine on Feisty, but when I upgraded to Gutsy I found I could watch the windows redrawing.
It turns out* that I had xserver-xgl installed from a previous (failed) attempt to get 3d acceleration working in Ubuntu. Feisty didn’t care, but Gutsy loads XGL automatically if it is installed. And XGL with no acceleration is really really slow.
* Anyone who has used linux/ubuntu knows what getting to ‘it turns out’ entails.