Hmm, I don't know of a *fast* knit afghan (I'm a slow afghan knitter!) but I do know crocheting an afghan (for me)is WAAAAY faster-I used a q hook and very simple pattern (using 2 strands held together) to whip up a good-sized afghan in about a week (seems like it took 4 days of evening crocheting). It was for my Dsis and dbil-they love it and she was so impressed (I was
inside because it was sooooo easy!).
If you're interested, I'll give you the pattern. I don't know if you crochet though, so I'm sorry if that's no help!