Does this occur with specific skins, or all of them?
I have noticed some strange behaviour similar to this. On some skins incorporating the viz, such as Thump, the cpu usage increases when changing the resolution. But this only seems to happen on certain resolutions and only if the viz is activated or visible. The cpu increase I get is only 3-10% and it goes back down if the resolution change is reversed.
Running Xion with Thump (box open) at 1280x1024 uses 0% cpu usually. Knocking the resolution down to 1024x768 results in a 5-10% increase on the cpu. Only reducing it to 1152x864 has no effect.
Testing out the same settings using Overdrive, which is more intensive, but doesn't use the viz, has no effect.
Extending the desktop seems to have only a small effect, I use Ultramon at times and everything becomes more complicated if its active, especially video overlays.