While I fully agree with the senselessness of this overlapping-parallaxing-animating background drivel I did notice it seemed to work rather well here, using about 52% CPU on an Athlon II P360-powered machine (not the fastest around) running Firefox 30 on Linux. This is still 50% more than I'd like to see once the page has rendered, but for that purpose I have noscript and friends - I actually had to allow scripting on that page to see the animations in the first place...