I am running Ubuntu 12.10 64-bit on a desktop computer that has 7.3 GB of usable RAM, an AMD A8-5500 APU Radeon Quad-Core processor, and dual monitors at 1920x1080 each, one through DVI and the other through VGA. The hardware is more than capable of intensive work, however, I've noticed that ever since I did a fresh install, Unity will sometimes become a bit sluggish while doing animations, especially after coming back from suspend. The sluggishness does not make the computer unusable, but the slower animations are not pleasant. When I first boot the computer, it seems to work okay as far as I can tell.
Normally, the animations are milky smooth, but when I suspend the computer and turn it back on, for example, the frame rate seems to drop quite a bit, especially if windows are animating across the entire screen or if I pop out the side bar, which is normally hidden. The lag in animation seems to affect the entire system, since the lag is also visible while using applications such as Firefox or Chrome.
To fix the problem, I either have to 1) entirely restart the computer or 2) go to my display settings, select one of my monitors, disable or change its resolution, and then hit apply. Once I hit apply, the one screen flickers off and then on again (if I just changed the resolution), and then the lag seems to disappear. I can also revert the changes, letting the screen turn off and back on again, and the lag still seems to be gone. Sometimes, though, I may need to do this twice in order to get the frame rate completely back to normal. Another thing to note is that if I only use a single monitor instead of dual, this problem never occurs, even after waking the computer from suspend. The problem also seems to only occur when I am using the screens as an extended desktop, not mirrors.
I first thought this issue could be due to a process utilizing too much of the CPU, so I ran 'top' but didn't notice anything very unusual about the results. While idle, the top processes are usually Xorg and compiz with about 0-3% CPU usage, and while windows are actively being moved and minimized/maximized across the screen, I've never seen Xorg go over about 30% and compiz about 15-20%.
Also, I don't know if this has anything to do with this problem, but when I bring the computer back from suspend mode, an error flashes on the screen for only about half a second:
[Firmware Bug]: cpu 1, try to use APIC500 (LVT offset 0) for vector 0x400, but the register is already in use for vector 0xf9 on another cpu
[Firmware Bug]: cpu 2, try to use APIC500 (LVT offset 0) for vector 0x400, but the register is already in use for vector 0xf9 on another cpu
[Firmware Bug]: cpu 3, try to use APIC500 (LVT offset 0) for vector 0x400, but the register is already in use for vector 0xf9 on another cpu
and it repeats. Again, not sure if it's relevant, but I thought I would mention it.
I now notice that Unity still does not work at full frame rate even when it is first booted. The only time it will work without lag is if I manually go in and change the display settings and then change them back.