This isn't a question about Ubuntu being laggy in general - not at all, in fact, it's very slick and fast for me. Clicking the "Workspace Switcher" in the dock performs the animation immediately and very smoothly. Switching between workspaces with the arrow keys - again, flawlessly.
My computer has a resolution of 2560x1440 on a 27" display (no, not an Apple product - though my monitor has the same panel that Apple use in their cinema displays). It's powered by an Nvidia GeForce GTX 470 - easily enough to handle it - and an Intel i3. Hardware is not the issue.
I am running Ubuntu 11.10 (upgraded from 11.04). I had the same issue in 11.04.
I'm running the "NVIDIA accelerated graphics driver (post-release updates) (version current-updates)" from the additional drivers dialogue. Two drivers have been suggested to me via that dialogue and I've tried both - same effect with each. The driver is "activated and currently in use".
Any other information required, let me know and I'll post it. I'm a programmer who works with Linux daily (both as a job and as an interest) so technical instructions are fine.
I've noticed that Compiz uses a lot of CPU when moving windows around and it's memory usage is relatively high (though possibly expected for Compiz):
1671 user 20 0 478m 286m 33m S 1 7.3 12:44.05 compiz
And one more thing - occasionally moving windows around is fast. But it only happens when all applications are closed, and even then it sometimes doesn't. Something must be interfering, but what? I'll try and find out but in the meantime, any suggestions are much appreciated :-)