I just upgraded to 12.04. I noticed a new weird behavior that did not happen on 11.10.
When I lock the screen, or when it sleeps automatically after the computer is idle, the CPU fan starts working hard (which usually means the CPU itself is working hard). When I return from the sleep to re-enter my password, the fan subsides.
I noticed some bugs related to compiz (Compiz makes CPU run at 100% when in suspend mode in 12.04), but I tried the suggestions there, and none seem to help.
Any ideas on how to check what's making the CPU work hard, or how to stop it from doing so?