I have a 6s. iOS 11 was pretty annoying and my battery drained noticeably quicker--I was bummed with the upgrade but I think I wanted it for AirPods support. Also I remember the "instant camera" was a feature demoed when the phone was first released and iOS 11 took 5-10 seconds to bring up the camera.

I've been using the 12 beta for months and it immediately fixed those things. I'm not sure if the camera is as snappy as the original iOS, but it's less than a second. I'm not sure how this all compares to the original iOS it shipped with, though.