It's 5-10 fps in Firefox Nightly for me, and 50-60 fps in Chrome Canary, on OS X.

OK. They're both about 20fps or so for me (at least eyeballing it).

Mind filing the bug, since you can reproduce and profile?

Edit: mstange did in fact file one: https://bugzilla.mozilla.org/show_bug.cgi?id=1300848

How do you measure fps? Is there an add-on for that?

In Firefox go to about:config and flip layers.acceleration.draw-fps to true. In Chrome I think it’s either in dev tools settings or in chrome://flags

Dragging the scrubber around updates at the same speed, really? I'm not on Nightly, maybe that's the difference, but if that's the case that's great news.