It's called Dojo2D.
Try this test page. On my machine, Firefox 22.214.171.124 will load that page in 8 seconds, which is about 7.99 billion clock cycles too many, for my taste. But Internet Explorer locks up for a full 30 seconds (consistently, every time) when trying to load the page.
I'm happy to see IE users brutally punished in this fashion, of course. But honestly, this has to be some kind of sick, sick joke, right?