I know, there is some forum common sense about Metal on iOS, but it might not be the (only) culprit. E.g. Mac OS Metal definitely improved performance. Animations are way better on my rMBP with El Cap. And Metal on iOS also improved gaming performance. So why should it suddenly destroy UI performance? Bad coding, lack of optimizations, it is all possible. But Apple also implemented in iOS 9 a different battery management (cpu/gpu speed stepping) and way more aggressive ram management (also idevices lack a dedicated VRAM for e.g. animations).
My Non Retina Macbook is improved with Metal and it wasn't even that bad before! I updated my Mum's rMBP the other day and there is a definite difference. The Retina MacBooks always drove me crazy with animations, but under El Cap, that seems to have improved. Sadly I can't say the same about the A7 Airs and Minis I've used under iOS 9.