Compiz starts smooth and fast, then bogs down

I just did a clean install of 8.10 on my Dell D600 laptop last night, and the radeon driver is working, and my desktop effects are enabled.

When I have no windows open (just a blank desktop), compiz runs like a dream. It's smooth, it's pretty, and it's really fast. However, as soon as I have any open windows, it bogs down really quickly. It doesn't matter what the window is--nautilus, xterm, firefox. As soon as I try any effect, the framerate goes down to the point where I can count the individual frames as the cube rotates. If I close the window, the performance returns.

Is this a known issue? Is there a fix?

Hardware:
Dell Latitude D600
Pentium M 1.4GHz
1GB RAM
Mobility Radeon 9000 (RV250) (why, oh why did AMD discontinue their driver support? :( )

EDIT: Looking on the progress page for the driver, there are several features which are still not complete. I don't know if any of those would affect the performance in compiz, though.

EDIT more: Lowering my resolution (to 1024x768 or 800x600) restores much of the performance. However, why would there be such a difference from a clear desktop to a single window?