If you're dealing with iOS 11 performance issues, the iOS 11.1 might be the remedy you seek.
Feedback from iOS 11.1 beta testers has been mostly positive. A few notable improvements in the iOS 11.1 beta include a faster unlock animation and a smoother animation when tapping on the status bar to scroll to top of the screen. Testers also are saying the overall UI seems to "be smoother and more polished."
We've been testing the iOS 11.1 update on the iPhone 6s and we've noticed some slight improvements to battery life and overall speed.
We encountered sporadic lag on the iPhone 6s when running iOS 11.0.2 (mostly when using the keyboard and pulling down Notification Center), but iOS 11.1 feels fast. That bodes well for the final release.
A lot can change between now and the final release, but we're feeling pretty optimistic at this point.
iOS 11 is pretty buggy (and sluggish on some devices including the iPhone 7 Plus) at the moment, but iOS 11.1 could help.