http://www.techradar.com/news/benchmarks-show-your-iphone-might-not-be-slowing-down-over-time

It is a truth universally acknowledged that as your iPhone starts to get older, it'll run slower and slower until you finally give in and hand over another bunch of bills to Apple for a new model - or is it? Not according to a new set of benchmarks from Futuremark.

The performance-tracking experts crunched the numbers in their database, based on real-world tests, comparing the iPhone 5S across iOS 9, iOS 10, and iOS 11 stretching back to April 2016. They also looked at newer iPhone models across shorter time spans, and across the board the results were pretty consistent.