Apple is the one company that cares about the finest details of their devices in both hardware. Every year Apple releases updates for iOS. For the past few years, these devices have actually made iPads and iPhones perform worse. No longer is the user experience guaranteed to be superb.

In September 2015, Apple released iOS 9. iOS 9 was said to bring performance improvements among new features. However, the opposite is true for a lot of people. For these people, it is clear that iOS 9 has slowed down every iOS device significantly. Even last years iPhone 6 and iPad Air 2 are suddenly much slower.

However, this is not new. The iPhone 4s was super smooth with iOS 6, yet in 2015 with iOS 9 it is much slower (https://www.youtube.com/watch?time_continue=171&v=9J6R0NnqOzc). The iPhone 5 used to be super responsive, yet with iOS 9 one can find himself frequently tapping the screen in between animations where nothing happens, other than waiting for apps to load and jerky animations to finish. 

This time it is different, because Apple promised performance improvements but every single device performs worse with iOS 9 than it did with iOS 8. Issues include: less responsive experience in general, jerky animations, stuttering in scrolling (for example the Twitter feed or Facebook timeline).

See these two threads for complaints:

 http://forums.macrumors.com/threads/so-turns-out-ios-9-is-actually-laggy.1916966/​

http://forums.macrumors.com/threads/ios-annually-slowing-down-our-devices.1926081/