Question to performance of multimonitor setup


I experience strange results, which I don’t understand.

I have 3 monitors attached to 2 Quadro 600 cards. 2 monitors are handled by
twinview and the 3. is merged in via xinerama. Composite is disabled.
When I run qvdpau test, it makes no difference, where I place the window. All
performance values are nearly the same and the values fit my expectations.

When I use testsprite2, the behaviour is completely different. As long as the
window fits in the twinview area, I get about 980 frames per second. When I
move the window, so that parts of it (or the entire window) is displayed at
the 3rd monitor, framerate breaks down to about 60 frames per second.

If I understand readme from nvidia right, all sli-processing is performed by
the first gpu, only the rendering is performed by each gpu. I thought, sli
should enable higher frame rates.

So is my expectations wrong or can I improve configuration or the like?