I also have an iPhone 4S. I wondered the same thing. It's really annoying. My guess is that the memory requirements have increased a lot. It kind of still feels

I also have an iPhone 4S. I wondered the same thing. It's really annoying. My guess is that the memory requirements have increased a lot. It kind of still feels fast after a fresh boot. But after a few hours, it feels really slow. Some bigger applications fail at the first run, maybe because too less memory. Some will crash when I open an embedded Safari page. Also, when switching apps, in almost all cases, it has to restart the app. Also, the battery duration has become worse. But that might also just be its age - although not sure. Now, just because of that, I'm thinking about buying a new phone. I wonder, is this similar for Android phones? When I buy a high-end Android phone now, can I expect it to still run fast in 3-5 years or so the future current version of Android?