We tried many things including disabling all background app usage, full wipe and restore, etc. My wife eventually took hers into the Apple store and they said they didn't know why it was slowing down but that she shouldn't have upgraded and that there was nothing they could do.
So faced with unusably slow phones, and no resolution, what other choice is there? We both upgraded. She bought an iPhone X and I bought a Pixel 2 XL. We would have both been happy sticking with our iPhone 6's as they were under iOS 10.
The crux of the matter is this: our phones were fine before iOS 11, then unusably slow. We upgraded because we were forced to (unknowingly at the time) by a software update.