One of my friends, The Cowch, has pointed me in the direction of Tornado, and even more bare-bones framework than Pylons. Sounds awesome! I'm going to give that a good shot first, and if it's too slow going then I guess I can switch to Pylons fairly easily. The most encouraging thing about Tornado is that FriendFeed is built with it, so there shouldn't really be any need to worry about speed.