I went back to iOS 8.4.1 on both my 6 and Air 2 last night, it's night and day just now.
I've no doubt that within a few updates iOS 9 will reach that level on newer devices but for now I'm sticking it out on iOS 8. I'll check out iOS 9.1 when its out of beta by visiting an Apple Store and trying out devices there to gauge performance.
The Air 2 is nowhere near as quick and responsive on 9 as it is on 8.4.1. What I loved about the the Air 2 was that you could pinch home quickly and immediately tap into another app, pinch home, quickly tap another app, pinch home, etc. It's incredibly responsive ... on 9 it was nowhere near so.
And the stutter at the very beginning of the animation when swiping down to activate spotlight drove me up the wall.
Yes, I have iOS OCD, but I notice any little frame drops or stutters and when I do it detracts from the experience of using the device. It's that simple for me.