Apple has been crippling older phones and tablets for years, anyone who doesn't believe this is absolutely fine to do so, but the evidence is in the fact that the assets used within the OS don't change, the images the processor and GPU need to throw around the screen don't change, and yet with each passing annual update the phone becomes slower and slower.
I had the misfortune of using a relatives iPhone 5c on 10.3 and my God it's awful. Delay between tapping an icon and it opening, delay when swiping to scroll and the scrolling starting, it's all there. Going back to my 7 was like injecting rocket fuel into my eye balls; speed!
I fail to believe that adding new "features" like fancy messages, transit to maps, etc affects performance of the ONE core element which does not change (from iOS 7 until now) and that is the assets for the UI. If those same icons, with the same drop shadows, and the same effects can be thrown around smoothly on iOS 7 then they should do so on iOS 10; there is no defence for it not being the case and the simple fact comes down to the one common denominator - Apple don't want you holding onto your phone for longer than two years, they want people on two year cycles, whether it's the 5 to 6 to 7 group or the 5s to 6s to 7s/8 (whatever it's going to be called).
And, finally, let's not forget the painful death of the iPhone 3G and iOS 4. No background wallpapers, no multi-tasking, just folders and the same UI assets as iOS 3 (with some mildly redesigned icons) and yet it crawled, it was unusable.
Don't believe me? Here's what Engadget had to say back in the day:
https://www.engadget.com/2010/07/22/ios-4-and-iphone-3g-is-a-match-made-in-whats-the-opposite-of/
As I said, choose not to believe it, but at the end of the day Apple isn't a smiley happy consumer focused start-up. It's a multi billion dollar business with a clinical CEO who cares about shareholders first and customers second; anyone who tells me the customer service experience in stores is superior or even on par to days gone by is kidding themselves on - unfriendly "geniuses", and the removal of Genius Bars from newly renovated stores and replaced with a "just sit up the back and someone will be with you" mentality, instantly come to mind.
I used to be such an Apple "fanboy" back in the day, I loved the company and has this ridiculous notion that the company cared about its users; and perhaps under Jobs it did (not making this a Jobs v Cook debate so you there, yes you at the back ... get back in your cage please).
Apple slow their devices down. That's my opinion. Don't agree that's cool, I can disagree with you thinking the exact opposite.