On current consumer devies, performance would be acceptable (30 fps, decent input lag) as long as you used only black and white. Grayscale takes significantly longer to display, though future devices may fix that.
I think that if you have a good grasp of the commands in an editor such as vim, having "real-time" feedback isn't as important as with other UI paradigms (as long as it's not too slow, you do need a few fps). Though I'm not sure. It will take some getting used to. The nice thing about E-ink is that looking a the screen is very relaxed, as it doesn't emit light. I'd love to buy an E-ink monitor to try out, when they are available and affordable enough.