PS is not actually very good at painting, its ok but its by no means incredible many much better drawing and animation application exist.
To be honest i dont know, i doubt it the animation interface has something to desire for. Tough this said Ive actually done animations with photoshop back in the days even before it did have animation capabilities (from photoshop 8 forward). Or rather it always DID have animation capabilities in form of individual files and filmstrip format. And no the performance actually was not all that much worse.
I would say that while ist not the best possible animation development package out there it is a decent colouring platform or at least significantly better than what it is in pure animation.