Andrew Cunningham, writing for Ars Technica:
I’ve been testing iOS on old devices for six years, and I’ve never seen a release that has actually improved performance on old devices. At best, updates like iOS 6, iOS 9, and iOS 10 didn’t make things much worse; at worst, updates like iOS 7 and iOS 8 made old devices feel like old devices. Anyone using an older device can safely upgrade to iOS 12 without worrying about speed, and that’s a big deal. You’ll notice an improvement most of the time, even on newer devices (my iPad Air 2, which had started to feel its age running iOS 11, feels great with iOS 12).
As I noted in my review, I was hoping someone would run actual measurements for different system features on older devices running multiple versions of iOS. Cunningham did exactly that, going all the way back to iOS 10 on the iPhone 5S.