Ok, did some extra testing on the desktop machine (which has a 7900 unlike the laptop which has a 9800M GTS graphics card, so less muscle) and the results are pretty interesting.
Here are some pre-made test caps, btw:
http://dl.dropbox.com/u/1328856/32x32.cap
http://dl.dropbox.com/u/1328856/512x512.cap
http://dl.dropbox.com/u/1328856/1024x1024.cap
So it's basically a tiled 32x32, 512x512 and 1024x1024 texture.
When the layout is power of 2 size the fps is pretty much identical on this system for all the caps ~800 fps. If the layout size gets changed to something more common like 1024x768 the fps get cut in half.
Copy-pasting the tiled background does the same, as long as the position isn't centered perfectly (in that case fps is about 70%).
If a non power of 2 texture is used the fps is about 90% of that. Which is strange.
Basically it looks like the GPU just really dislikes anything that isn't power of 2. Be it texture or layout size. It's happiest when it gets to just render what's inside its memory straight up, no cropping. Also the performance seems to vary quite a bit among graphics cards (logical, of course).