I think it's because of the heavy footprint of the background image that's forced into the footer. This might cause a lag in scrolling on some operating systems / browsers. Personally I do not notice this on my 24" iMac for example in Safari or Chrome. But I do notice it in the 2500x resolution on vmware > internet explorer or firefox on Windows XP.
The final 2012 version of the xenfans style that is being played with on my localhost has cut this lag speed down by 45% I will escalate that code to the 2011 style, hoping that xenforo will go stable this year.
It will have a smaller footprint footer bg, with 16 colors, and a more complex dynamic footer-template background image to populate it. This fixes various design issues, alongside a bit of the scroll-lag.