I met the same problem as yours and fixed it this way :
When you log in fedora 12 gnome or XFCE with compiz enabled, the speed is dramatically slow. To fix this,go to compiz-fusion icon, click on the right mouse button, choose "Compiz Options" and enable "Loose Binding" and "Indirect Rendering". After this my compiz on gnome and XFCE became very very fast.
Hope this will help