Clean reinstall might help somewhat Luca. It did with with my iPad 2, which is running iOS 9 quite smoothly.
I have some other difficulties with Apple, but I think that Apple actually does a great job in backwards compatibility, within the limits of what is reasonable. Sure an older machine is not going to be as good with the newest software as a new machine, but my old iPad 2 is still very functional. I did treat myself to the Air 2 recently, but the iPad 2 is still giving constant service in the hands of my son.
When there's an eel in the lake that's as long as a snake that's a moray.