I’ve slo-mo videoed iOS 11.1 running on my iPhone 6 and while it is faster and smoother than 11.01 I believe the problem lies in Apple’s new animation changes. They seem to require more memory and cpu speed because each time a transition from one view to another happens it basically saves the old view, probably as a layer, then zooms in the new view under it and slides the old view out of the way. It’s supposed to look better, but really it uses more cpu time, memory and therefore slows it down. Not good for older cpus. I believe 11.1 and subsequent updates will improve the speed and smoothness on the view animations. This does appear to be a case where the new iOS version is really designed for or around the new models. My iPad 10.5” Pro works like a dream with 11. Right now I’m planning to upgrade to the X by Spring 2018.