With the following setup :
- Ubuntu 10.10 beta, xserver 1.9
- nvidia drivers 256.53
- GF 9800M GTX
Display is *extremely* slow (scrolling in a web page, moving windows, etc.), with & without composition. Actually it turned out this is due to the antialiasing text rendering (games & videos are still smooth).
It didn't occur with the same drivers in Ubuntu 10.04.
Here's a relevant bug report : https://bugs.launchpad.net/bugs/629910