How to Downgrade From iOS 11 Beta to iOS 11 Without Factory Resetting
It's been a while since I did this but if I remember correctly, go to beta.apple.com on your phone and login. Go to the IOS section and un-enroll your device from the beta program. It should then allow you to download the non beta updated version of IOS 11.

It is likely the same build that you are running now. I am not sure if beta builds have extra debugging code in them or not. If this does not fix it you may need to back up the phone, erase it and restore from backup.