I didn't get a chance to dig into what, but looking at a quick profile it seems we're running an animation on this page that ends up consuming tons of CPU time (in both Firefox and Chrome), yet nothing is visibly changing on the page (that I can see). Maybe there's an optimization opportunity here where we could avoid some work?
As far as I can tell we have no such bug for SVG animation but we have already one for CSS animations/transitions or script animations. Bug 1237454.
I am going to re-use this bug as the one for SVG. This might be fixed once we integrate SMIL and web animations code in Stylo.
Oops, I am sorry. According to the test case in comment 1, animations on the site are not on visibility:hidden elements. Moving back to core::layout.
Mass wontfix for bugs affecting firefox 52.
The animation in the test case in comment 1 is transform:rotate, it produces nsChangeHint_UpdatePostTransformOverflow, so even if the animation is scrolled out of view, it can't be throttled. That's said, if the target frame height is zero (actually it's the test case), we can throttle it, as far as I can tell.
The slideshare case will be fixed by bug 1190721.
Moving into SVG since the CSS animation case in comment 1 has been solved by 1190721.