Nominating for inclusion in 86 work, but feel free to bump to 87 if needed.
The tree integrity looks good. I suspect this is an issue with AXRole depending on querying content for "is layout table". I think we need to figure out a way to avoid that entirely. Morgan is working on that in bug 1680840.
Just tested this locally after applying the patches in bug 1688730 and this is still a little slow. I'm not sure if I have a good heuristic for how slow gmail is normally, though, so this is probably worth having :marcoz test again on top of that patch stack once he's back :)