Watched the video, and that definitely is weird. At first I thought for sure it was the same dropped keystrokes issue, but clearly none were dropped. I have no explanation for this. I also have no explanation for why the cell properties dialog would work normally. They're all using the same type of control, a simple HTML <textarea>. Actually, the cell HTML input has additional handlers on it, which, if anything, could slow it down. Try pasting a LOT of text into a the cell HTML input and see how slow it gets (I fixed that in 3.4.b2 or b3 however).
I wish I could offer something, but I got nothing. Of course, you can always try the beta version, but if you've already applied the mshta.exe fix, I don't see how it might be any different.
Keep us updated if anything changes.