As someone who tested this extensively, it really needs to be treated like a major OS upgrade on a computer. Back up important info(pictures, etc) without using icloud, do a full format(DFU restore), and reinstall from scratch.

I have an iphone 6 and 6s here that are still flying along great on ios 11. And i had previously had so many problems with the 6S that i thought this upgrade really was awful. It took until 11.1 or so to truly be ok, but the performance increase from doing a true "clean install" like i would with windows or OSX was really night and day.