Re: Poor Xinerama Performance
I have been experiencing this problem, too. In my case, it's an AGP FX 5200 and a PCI GF2 MX200.
I have a couple things to add:
1) TwinView (just the 5200) works perfectly.
2) Non-Xinerama (old-style multiple X screens) works perfectly.
3) OpenGL didn't appear to be the issue, as I wasn't running anything with OpenGL, removing glx from xorg.conf had no effect, and I wasn't getting the OpenGL error mentioned above.
Hope that helps someone. I've decided turning off Xinerama and using the old X style is good enough for me, so if you're having this problem, you might want to at least give it a try.