I've had it pointed out to me that there's something weird about the show Gilmore Girls in the actors' delivery of lines. The actors don't seem to be speaking particularly fast, but something about the structure of the dialogue still seems to be perceived as very "dense" by most people. I'm guessing the effect is from the actors shortening the conversational gaps between their line deliveries. It seems like the each reply comes near-instantly after the actor "heard" the previous line, which may be a super-stimulus for making the portrayed characters seem extremely "witty."

I also have a feeling that actually increasing verbal velocity would hinder this effect; you wouldn't get the impression of a witty character, but rather a manic one.