After upgrading my iPhone 6S to iOS 11.x, I noticed it was really really slow.
Each keystroke took at least 0,5 to 1 second, opening the camera app took 5-30 seconds.
I searched a long time to find a solution, but the web is mostly full of complaints without answers.
Then I found this discussion on Apple’s discussion boards:
Serolio states as the first helpful solution, and it worked for me:
Oct 11, 2017 9:03 AM
„HERE’S THE FIX: Go into Settings> General> Reset> Reset All Settings (this will reset iOS settings, won’t touch your apps)
WHAT THIS DOES: This will *not* erase your phone, but will reset most of the things you can adjust in settings on iOS (you’ll need to re-enter you wifi, adjust notifications and privacy settings again, reset your default account for Calendar and Notes, etc, but all your email accounts and iCloud account will still be logged in and all working).
FIXES THIS ISSUE: I had the same issue: iPhone 6S. Upgraded. Even with 25GB free, I’d press a button and often have to wait 1-3 seconds for a response, or copy and paste selectors would be really slow to appear, even the keyboard was unresponsive… phone was barely unusable, literally. I think most people don’t get it because most who updated to iOS 11 noticed it being a little slower but not 90% slower. Those like us need to take this step.
BACKGROUND: When I was scouring I found that erasing your phone and restoring from backup worked for people on Reddit, and that should be reliable. I was in the process of doing that myself and accidentally chose „reset all settings“ instead of „erase all content and settings“. My phone rebooted and all my apps are fine, just needed to go through iOS settings and readjust things.
ALTERNATIVES: This is obviously better than either (a) downgrading to iOS 10 or (b) making sure your phone (as it is now on iOS11) is backed up to iCloud, erasing it, and restoring from that backup.
The difference in speed is shocking.
Pretty sure this will work for you too.“