It would be really great if there was a way to have various layers animating at a unique frame rate. So, for example, if you had a waterfall scene you could have the waterfall flowing much faster than the calmer lake that it feeds into.
Its true that you could have the waterfall animating on ones and the lake animating on twos or threes, but then you have to make extra frames for the waterfall to loop through in order to catch up with the frames of the lake. For every one loop of the lake the waterfall would have to loop twice or more. It would be a lot more streamlined to just have the option to set up varying frame rate per layer.