Here’s a bit more information on this bug (now that I’ve had some time to remember) …
It’s definitely a bug in the underlying macOS text system. I reported it to Apple Dec 2015 and it was marked as a duplicate of bug #23823164. I just checked and that bug is still open (not fixed) and unfortunately I have no way to get any other information on the status of that bug.
The underlying problem is fairly easy to reproduce outside of TaskPaper, for example I get the same super slow typing experience (on my very fast and new iMac) if I follow these instructions:
From what I can tell the underlying problem seems to be that the text system is invalidating the entire text layout for all edits. In “normal” text it generally only invalidates the current paragraph for each edit. But in some situations it starts to invalidate much more. And that’s what’s causing the problem. But why it’s doing all of this invalidating I don’t know.