I recently upgraded my work PC from Natty to Oneiric, and discovered that whenever I had a window open that wasn’t maximized, it’s backend process and xorg would use up 100% of the cpu.
I was nearly about to rebuild my machine when I discovered that it only happened for non-maximized GTK windows. As a last ditch effort, I ran gtk-theme-switch2, and switch my theme. Magically, this fixed the problem!
Part of the issue was that I’m running in Fluxbox, not Gnome or Unity. I think somehow it got the theme messed up during the upgrade, and since neither Gnome nor Unity had the chance to set it to a correct theme, it borked.