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).
In theory this sounds like a great idea, but it could be difficult in practice to find the right balance. I think there are some animations, which seem to improve greatly (like the folder opening, spotlight) when they have been used more than once. So it might be easy to point the finger at Metal while there could be other variables at play. So far we don't have any proof of what is really going on and therefore i suggest to continue writing bug reports until this is sorted out.