I think part of it is software upgrades, many of which are optimized for newer hardware, or not optimized for out of production hardware. So when new features

I think part of it is software upgrades, many of which are optimized for newer hardware, or not optimized for out of production hardware. So when new features are added/enhanced older systems may see a slowdown, another factor is newer hardware runs faster so some new updates may not care about optimizing as it runs fine on the newer system anyways. Have used Apple products for a while, they don't really like spending a lot of undue effort on stuff they don't sell anymore. Still running iOS 8 on an iPhone 5s, works great, I hear newer iOSs are not as efficient on this phone, so will stick with 8 on it.