Since processingjs is terribly slow on the iPhone's webview with the animations I tested, I'm looking for an equivalent of http://wiki.processing.org/w/Android for iOS, that is, hardware acceleration via OpenGL -- maybe a game library like cocos2d-x but I couldn't find one that understands Processing.org scripts.
I recommend having a look at Davide Della Casa's P5Nitro project. I've seen a spring based sample running on both P5 Nitro and iProcessing and the P5Nitro version was much smoother.
Also, I think you should try OpenFrameworks, a c++ project inspired by Processing (loads of familiar functions already supported) which can also target iOS.
In conclusion, try P5Nitro and see if it does the job for you out of the box, if not, have a look at openFrameworks and try to port/chance your Processing code a bit. There's a nice little guide for Processing users available.