This is not limit to Apple iOS.

Android has this problem too. If you upgrade your 2 years old phone with the latest Android (if your phone manufacturer did happens to provide update), your phone is not responsive at all. I know, few years ago I updated my Note 1 to Lollipop and it was completely unusable. The good thing is the Note 1 has unlock bootloader and I was able to restore the older software.

Unfortunately, you cannot downgrade iOS. I wish I can as I am on 11.2 and the battery drain is crazy. 10% on idle. FU Apple.