To add to ricardobeat, it's all repaints and recalculations. Web devs have become overzealous with smashing JS into every nook and cranny to manipulate the dom,

To add to ricardobeat, it's all repaints and recalculations. Web devs have become overzealous with smashing JS into every nook and cranny to manipulate the dom, it seems to be a battle Google is trying to educate people about, and good on them. Open dev tools in chrome, the click network/frames, hit record and refresh the page. Do this on a sluggish site that uses tons of js and you will see the problem.