My iPhone 6s suffered similarly on iOS 11.0, but is back to normal iOS 10 level performance with 11.1 Beta 5. I would try the beta if you're impatient, or just

My iPhone 6s suffered similarly on iOS 11.0, but is back to normal iOS 10 level performance with 11.1 Beta 5. I would try the beta if you're impatient, or just wait for iOS 11.1 to come out, which will probably be released within the next 1 to 3 weeks. You can sign up for the public beta at beta.apple.com, which involves installing a beta profile on your phone, rebooting, and visiting Settings/General/SoftwareUpdate and starting the install. You can later unenroll your phone from the beta train by deleting the beta profile in Settings/General/Profiles. Generally, early iOS betas, even public betas, are awful. I wait until at least beta 3 or 4, as showstoppers and most minor bugs will have been fixed. If you still have your 6s next year, I'd skip 12.0, and wait until 12.1.