I`m not sure if this is still true with AFS and do not forget the security aspect. All apps get updates, unless they are not to old. All our devices are up to date: iPhone 5, iPhone 6, iPad 4 and an Apple Watch Series 0.
Absolutely true, even with AFS. That's why I am still on 10.2 and I am perfectly happy with the speed and battery life (7-8h of usage, 30+h standby, 6S). Speed-wise 10.3.2 is not better than 10.2, read/write performance is even worse on 10.3.2 (a bit faster system animations on the last and that's it)