I've just bought an iPhone 6S second hand with a new battery. I don't know how fast they used to be, but I've not noticed anything being slow. Animations are silky smooth, and app loading times seem fast too.
That's the problem. :)
I'm also on a 6S and Apple has kept it quite speedy! But, I was playing with a 2016 iPhone SE on iOS 9 about a year ago, and I could instantly tell it was snappier than my 6S on iOS 12.