Unwrap the newest iPhone 7 or 7 Plus, and on the latest iOS version both will feel buttery smooth with zero indication of lag or a slowdown. It's a cycle every year: the newest iPhone will undoubtedly perform flawlessly on its respective iOS version that was released alongside it for at the least its first few months. Then, magically as if it was by design it'll begin to exhibit the slightest hiccups and holdups just doing menial tasks. Sure, Apple does add new features every year to iOS that use up more resources. It's not expected that an iPhone 7 that feels absolutely smooth in every aspect today will feel the same way in five years. However, having owned every single iPhone I am confident in stating that it's doubtful it will perform or feel the same in a year from now when iOS 11 is released. My iPhone 6s Plus like my iPhone 6 Plus before it slowly started to lag past the point of usability so much so that I had to do a factory reset. Even with a factory reset though, it still is not as buttery-smooth as it was on iOS 9 the year before. Frames are noticeably dropped even doing a UI task as basic as using Force Touch. It's absolutely expected that Apple's newest flagship phone is buttery-smooth and that the latest iOS version is optimized for the best experience on the newest phone. But the question is whether Apple purposefully neglects to optimize to retain that smoothness on older devices. All that is immediately apparent is that Apple creates the best experience for its new software versions around its newest devices, which is totally fine. If you want the best iOS 9 experience, buy an iPhone 6s. If you want to use the newest iOS 10 to its fullest potential, buy an iPhone 7. Just don't expect either to perform as well as out-of-the-box on newer software versions after a year, even with a factory reset. Apple's not the only one doing it, and updating to a new phone every year isn't a huge hassle provided they continue adding a myriad of desirable features annually.