A good chunk of this is layer building

A lot of the problem here is that we keep triggering paints even though the animation is usually offscreen. I'll look into fixing that eventually, but this is a nice testcase for slow display list/layer building in the interim.

Is the STR here just to load the page? Or scroll around as well? I'm trying to get a before/after profile to see if bug 1205087 comment 17 helps but I don't see this page hogging the CPU.

Just loading the page should be sufficient. The page is painting at 60fps for me when I load it (but probably not using 100% cpu) FWIW, I believe this was the test page that Matt was using for bug 1205087 and he noticed a large change.

Ah, thanks. I was grabbing the profile right after page load so the page load scripts were drowning everything out. If I let it sit for 30 seconds after page load and then grab the profile I see displaylist building taking around 34.8% of the time and a PLDHashTable::SearchTable taking around 3% of the time. I'll see if improving the hash key brings that down.

Goes down to about 2.5% which is not really that helpful. Overall the displaylist building is roughly what it was before.