It’s a bad thing that Apple doesn’t allow older versions to be downloaded. Firefox Focus was there before iOS 11 too, though it’s not as feature rich as the latest one. If you had downloaded it on iOS 10, you could still continue to use it or redownload that version.
I had an SE, 11 didn't slow it down. 12 was made to be faster than 11 even on the oldest supported devices. The SE has the internals of the 6S, only a couple generations behind, so it'll be fine.