I was pleased to see this in the ChangeLog for the nvidia drivers 256.53:

"Fixed a bug that caused extremely slow rendering of OpenGL applications on X screens other than screen 0 when using a compositing manager."

Well that sounds nice. Hands up who runs compiz on an nvidia card with two screens using xinerama? You might notice your GL screensavers run incredibly slow. Here's some examples, for testing I used glmatrix which you might be able to run directly via /usr/lib/misc/xscreensaver/glmatrix.

rest here