This slowdown has happened consistently in every release since iOS 6. Apple just optimizes the experience for the latest iDevices, which are always blazing fast, but then they slow down drastically as the years go by.

This is either an engineering decision (let's just focus our efforts on the latest thing) -- but then optimizing the whole iOS would speed up ALL of the devices, wouldn't it?; an economic decision justified as an engineering one (who cares about the old devices? let's just sell them a new one); or incompetence.