My own iPhone 7 is plenty fast, but I have noticed that the older models seem to be getting slower. Presumably this is because Apple keeps adding features and

My own iPhone 7 is plenty fast, but I have noticed that the older models seem to be getting slower. Presumably this is because Apple keeps adding features and animations and so on, and so iOS grows bigger and more resource-hungry, as are apps, and developers aren't optimizing by running their stuff on old iPhone 5 models anymore.