Typing into long documents is very slow.
For example: https://en.wikipedia.org/wiki/Intercollegiate_sports_team_champions?veaction=editsource (reported here: https://www.mediawiki.org/wiki/Topic:U8kj6nbqxpc982gy)
This has 400k+ characters across 10k+ paragraphs. On my machine, typing a letter anywhere in the document takes longer than a second.
Quick profiling reveals that most of the time is spent in ve.dm.TreeCursor.prototype.checkLinearOffset. This function is apparently called twice per paragraph, and it adds up the lengths of all paragraphs preceding it, thus the quadratic performance.
That was https://gerrit.wikimedia.org/r/#/c/415871/ (and a little bit of https://gerrit.wikimedia.org/r/#/c/415867/)