Both my wife and I have three year old 5Ss, but our experience with iOS 11 has been completely different. For me it works just as well as 10, maybe even slightly better. For her it has turned her phone into a complete dog, even opening a contact in Messages takes seconds.

Recently her battery has started to show signs that it is wearing out, such as jumping instantly from 5% to 30% when plugging it in to charge, so it looks like this behaviour is a software 'feature' on all iPhones.