I'm starting to think that having old versions of an OS running is not a bad thing. Going to iOS 11, I see a huge decline in performance on my iPhone 6. I assume that it's pretty difficult to create an OS meant for new hardware but still able to support old hardware efficiently. JMO If i could go back to iOS 10 I would.
I have four daily-use iOS devices. The iPhone 7 is on 11.3, but my Air 2, Mini 4 and iPod Touch gen 6 are all on 10.3.x. I can see the difference between 11.3 and 10.3.3 on the Air 2 because my wife has an identical Air 2 running 11.3. My personal adoption rate is only 25%.
With respect to the iPhone 6, after eliminating (by replacement) possible battery issues, my wife's iPhone 6 was so slow in so many ways that she gave up and replaced it with an 8. (But the iPhone 6 was the last iPhone with only 1GB of RAM.)