I went to https://public-artifacts.taskcluster.net/NdeLecVyS46y632NkRvlaw/0/public/logs/live_backing.log and scrolled down a few lines, then hit page down 2 or 3 times, and got a several second hang. So I grabbed the profile https://clptr.io/2lmDkg5 which shows 6 seconds in DoRemoveFrame. Half of that time is in nsLineBox::IndexOf.
I think this is doing a linear search and we have existing bugs on it, not off the top of my head though. There is bug 1085113, but that sounds like it would only make things faster if its always looking for the first or last frame.
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