Exactly what I said. It runs "fine" on 7, while it runs slow(er) on 6S.

iOS 11 runs fine and just as fast as iOS 10 on my SE. It sounds like you are experiencing a bug.

Aren't there, like... 3rd-party chat or emoji apps or something now? I don't use any of that stuff, but if only some people on 6-series iPhones are experiencing a multi-second delay opening the iMessage editor, my first guess would be they have some of those installed and one or more of them have shit the bed.

Ah, my mistake, I was taking the scare quotes to mean you were complaining about it on your hardware.