Make it more responsive. Especially typing should happen without noticable slow-down.
I'm a newcomer, as of July 2012, and I've noticed some oddities with typing speed as well. Deleting, too, with the backspace key, can be a little tricky at times. The cursor appears to stop, but when you let off the key whole chunks of text disappear. It's not a dealbreaker by any means, but it is a little annoying.
Yes! It gets pretty sluggish on my machine. I'm tired of having to split my outlines into several files just to make the performance workable.