I've gone through a few of them since buying mine 6 months ago, and yeah it's going to be slow unless you manage to get one with iOS 10 on it.
In stark contrast is my Moto X2, after 2 years of heavy daily use before my current phone, and one screen replacement is still chugging along pretty well on its archaic version of Android.
The argument that iPhones have superior hardware and build quality resulting in longer usage is negated by this constant push for upgrades. I think I know where I am headed for my next phone.