Yeah, I'm almost positive you're right about the animations, etc. still loading off-screen. I'm not sure how much of this is due just to javascript stuff going on vs. loading a bunch of things at once simply being really CPU intensive.

EDIT: Might be worth investigating the (dis)allow-scripts variable of the sandbox tag? Take a look at http://www.html5rocks.com/en/tutorials/security/sandboxed-if...