I spent some time today looking into the performance issue again. All I came up with is that disabling physics helps a lot! (How obvious is that?) I couldn't necessarily rule out collision detection as the culprit. I was able to rule out drawing operations as a problem; they are all incredibly fast!
In any case, more than 10 bodies and more than 10 constraints in the space is enough to start slowing the simulation on iPhone 4S. That's a very unfortunate limitation that I'll probably just have to skip for now. I'll come back to it later for sure, because I want to do mobile games, too.
In other news, my partners in this project have gone silent, so I guess I'll be lone-wolfing it. Dunno what I'm going to do with it, however.
Tonight's build only has an updated copy of Chipmunk-js, which fixes a bug in the collision callbacks (not that I'm using them yet, but I ran into it while developing Neverwell Moor, so I'm glad it's resolved now!)
One thing I need to work on soon is the README. I'll just put some basic details about what the game is supposed to be, and hope it can get to that point by the end of the month.