I think I’ve nailed down the bug:
Again, no full transcript, but it boils down to this: when editing large file in CFEclipse, there is a slowdown that occurs while typing where the next character (to the right of the cursor) is a left or right angle bracket. Insert any other character between the cursor and the bracket, even a space or a newline, and everything is back up to full speed. Since angle brackets are slightly pervasive in HTML and CF, this is more than a little annoying.
Update: And yes, before anyone asks, I have tried turning off the auto-complete. Even with it off, typing up against brackets is hella slow. (It is a slight speedup, though, as everything after the first word of a tag is thus fast, as long as you aren’t pushing up against another angle bracket to your right. Which, given HTML and CF, you probably are.)