Also, looking at their own performance numbers, it appears that all of this only managed to reduce the character insertion latency by 21% when compared to CLion. That's still useful, but maybe not even noticeable to the user.

That latency can’t be render latency, because it’s trivial. It has got to be related to unifying the text buffer with the syntax tree. Otherwise you’ll get wrong syntax highlighting for a couple of frames and that’s jarring.