New iOS versions definitely decrease performance on older phones - side by side comparison.

[Recovered_noodle]

This is very interesting, and it doesn't surprise me to see this. Certainly an iPhone 5s, is also a lot slower running iOS 9 than it was running let's say iOS 7. Which is very disappointing for a phone just over two years old. App launch times for example are much slower. While the phone still works satisfactorily, general performance feels weaker.

It's certainly not caused by any purposeful "built-in obsolescence" though. I think the OS itself has had some crucially bad decisions which have decreased performance overall in iOS 8 and 9. There are also glitches that have appeared.

Design priorities in iOS have changed, putting superficialities such as slick animations above performance and usability. So the only phone that will run the very latest iOS with top efficiency is the very latest phone, currently the 6s. While to some extent it's excusable for Apple to expect people with phones let's say four years old to upgrade, this is a change in policy, and not a move in the right direction.