It's because there's a wrapper div with "overflow: auto" around the entire page. This means you're scrolling inside the div (unaccelerated, need to repaint) in

It's because there's a wrapper div with "overflow: auto" around the entire page. This means you're scrolling inside the div (unaccelerated, need to repaint) instead of scrolling the entire document (accelerated, no need to repaint, can just move around pre-rendered tiles). Removing the "overflow: auto" makes scrolling smooth.