I’m not saying I wouldn’t expect some slowdown but Apple essentially forces you to upgrade with no path to downgrade the os. After upgrading my iPhone 6 to iOS 11 I could not launch Lyft, uber took minutes to open, and even the built in message app would have multi-second lag on key press. If you are destroying what was a perfectly fine phone with a software release it should simply not be supported. I would have been content with an old version of iOS that had reasonable performance and did not result in my phone shutting down at 30%, which was a problem introduced with iOS 11.
I wish someone would take the time to benchmark how stark the performance difference is between Day 0, Day 365, Day 730, etc. of doing the same exact tasks on a factory reset device with whatever is the latest software version at that time.
It's not hard to roll out new features behind flags for devices which can't reasonably support them. But your experience of ridiculous lag trying to open basic apps which have no particularly high resource requirement has been my experience exactly.
There's more going on than simply, "we added new features!" Devices which on Day 0 are blazingly fast and incredibly responsive because literally unusable in a few years doing exactly the same tasks.
Well yeah, since more features mean slower performance.
I saw a noticeable performance decrease when I went from Windows 7 to Windows 8. That doesn't mean Microsoft started throttling the CPU.