As speed tests conducted by YouTuber iAppleBytes show, Apple has managed to considerably improve the performance and benchmark scores in iOS 11 since the first beta. While the performance of most of the newer iOS devices is similar to that of iOS 10.3.3, older devices like the iPhone 5s, iPhone 6, and iPad Air do show a small dip in performance.
Even on relatively devices like the iPhone 6s, iOS 11 does not bring about any performance improvements. Knowing Apple’s past track record though, I guess users should just be happy that the performance of their iPhone will not decline considerably once iOS 11 lands.