Hi,
first time trying out Nexuiz, but heard a lot of good things about it.

Anyway: iMac (early 2009), 24", GeForce 120
Nexuiz in 1920x1200, high video, ultra effects. I play the first round against bots - 130-150 FPS, looks beautiful. I quit Nexuiz, restart, same level - suddenly total performance drop to 10-20 FPS. Unplayable. Same graphics quality, as far as I can tell.

SDL version does not make any difference.
Where does this come from? No crashes, just running slow.

Another thing is that the mouse feels weird. I see that's a complicated issue, it seems the only game on the Mac that got it right is Quake3. UT2004 for example also fails imho, even though not as bad as Nexuiz.

Hard to say.. does it 'work' again after a restart? Could be some driver bug maybe?

The mouse issue might get better if you try the 'OS mouse acceleration' option in the input menu. Or maybe try gl_finish 1, that sometimes helps with input problems (0 is default)