I finally got around to testing the delays while in a new "Compose" box. It was fine. I then went back to another file in the Inbox and replied to that. The delays were as bad as ever - equal to writing five words sometimes.
Does that give any clues? The idea of a buffer not coping does seem plausible, but I have no idea what to do.
It's funny, but I got to like the chain logic, so worst scenario I'll have to paste in from a virgin box. Don't want to have to open Word or somesuch.