Eh, it depends on if people mind a lot of very short posts, which is what makes a rapid conversation flow quickly.  Eventually you're going to end up using filler narrative you've already stated because the chars are saying more, but nothing new is really happening.  *shrug*  Granted, it helps to have time, but you can also post back-and-forth in a doc just as fast as on a forum, and that way you can also edit back and forth easier.  It's when you need to discuss things with the players that it gets messy.