Hi,
I am still using Fedora 15 and still having trouble with dual displays.
Immediately when I connect an external display, CPU usage of "compiz" goes up and everything else gets slow.
OS: Fedora 15
Desktop: Gnome 3.0.1
CPU: Intel® Core™ Duo CPU T2350 @ 1.86GHz × 2
Graphics: Intel® 945GM