Wow, is all I can say! Big difference over the last version of iOS 11. Reckon I can easily get another year before upgrading now. I’m keen to see how a 5S performs just out of curiosity.
I installed it on my old 5S to compare with my 6S running 11.4 and was surprised to see that in day to day usage they were pretty neck and neck. Obviously the 5S lacks the processing power of the 6S so anything that requires any heavy lifting is much faster on the 6S, but the basics of opening apps, navigating around the phone and so on are pretty identical in performance. For example, opening the Maps app is just as fast on both devices, perhaps even faster on the 5S, but loading in and processing the actual map content is significantly faster on the 6S. I also experience none if the lag on the 5S that I experience on the 6S when navigating around the home screen.
Put simply, the basics have been sped up but there's no substitute for raw processing power.