I have noticed that doing animations on windows in IE7 and 8 -thats right, not IE6- and on mac in Firefox on top of background images (such as on this page: http://thecreatorsdocumentary.com/artists) results in some pretty choppy animations.

If I remove the background image (recommended method is to use this bookmark: http://erkie.github.com/ and point at the top left of the page before blasting) the animation is smooth as butter.

Does anybody have any suggestions for improving this?