I notice my iPhone 5s is slowing down on iOS 11, I perform hard reset or install fresh iOS from iTunes and use the setup as new, no backup restores. IPhone 5s act just fine for a few days and then it's become slower again. I repeat the same process "restore" "fresh install" and it's again slowed down. Every time after restore and fresh installation iPhone 5s perform very slower and leggy after 1 week. I just use 3 third party apps, whatsapp, Facebook and messenger.