The iPhone 7 and iPhone 7+ have been plagued by a slow boot problem, usually after a screen repair. There are a list of culprits that could cause this problem. It seems as though after the iOS 11 update, the software checks all of the major hardware on the iPhone and if there is a problem detected, then the phone goes into a very sluggish state, taking forever to boot and when it does boot up to the home screen, the phone sometimes freezes.