Hello everyone,
Welcome to Apple Support Communities, for everyone who has slower than expected performance on their iPhone or iPad, in the post we have found many methods that work.
1) Update to the latest version of iOS 11, which is iOS 11.1.1, this update brings big fixes and improvements to your iPhone and iPad:Update the iOS on your iPhone, iPad, or iPod touch - Apple Support
3) Restart your iPhone or iPad by: https://support.apple.com/en-nz/HT201559
4) Reset iPhone Settings, go to Settings > General > Reset > Reset All Settings, this will not erase any documents or data, but backup your device beforehand.
If nothing else works, restore your device to iOS 11.1.1 using iTunes, you can learn how to Restore your device to factory settings using the provided articles below, then after you have restored your device, you can restore from an iCloud or iTunes local backup after. Be sure to make an encrypted backup so Health & Activity, iCloud Keychain and password settings get kept after restore.
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
Thanks for using Support Communities
RANT:
If the steps provided here, don't work don't complain to us about planned obsolescence, or Apple slowing down iPhone's, because I have had a positive experience all throughout iOS 11, and it's not the latest device, also don't recount your personal experiences then blame Apple for your misfortune. Do the smart thing and take your phone into an Authorized Apple Re-seller and get it fixed. Stop complaning!
<Edited by Host>