A consideration for those debating whether they need to upgrade a 6S. iOS11 seems to have improved the speed of my phone. Upgrading to 7/8 seems like it would be more a battery/camera decision than anything else.
I bet the 6S is still more than capable, but I notice some slight stuttering/delay when using force touch on an iPhone 7 on iOS 11. Wish I didn't notice these stupid things.