I'm quite frustrated by the performance of iOS 9 on the iPhone 5s. There's stutter all around the place and framerates are generally really low. Scrolling in settings or Twitter feels very jerky. Opening control center and spotlight is superlaggy. Weird thing is though, the iPhone 5 seems to actually perform better than the 5s. I carry them both and the difference is really huge, animations on the iPhone 5 are supersmooth, smoother than the 5s! Truly odd. I don't know the specifics, but I think it has something to do with metal that does not seem to perform on 64-bit chips as it does on 32-bit chips. However, I think it's unacceptable to release an OS update that slows down your phone this much. My 5s has gone from super "why would I want a new phone" smooth (8.4.1) to super "this phone feels very old" laggy. Just unacceptable and I'm really disappointed.