Sorry for the late response!
First step is to determine if you limited by draw calls or CPU or fill rate.
How is the rendering done? Is it batching all the rendering into a single draw call? How many atlas pages images are in your texture atlas? Ideally it has one, else you could be having many draw calls.
Can you show the Metrics view in animate mode for one of your animations? If you have many deform keys or many timelines it can increase CPU usage.
The number of slots should not have a big effect on FPS, except that you draw more pixels. The Metrics view shows the area (in pixels squared) being rendered. How much is too much depends on your video hardware.
Depending on your needs and where your bottlenecks are, you could update the animations at half the framerate to reduce CPU usage. Eg, update half your skeletons every other frame and the other half on the other frames. Another way to cheat is to pose a skeleton once, but draw it multiple times. Eg, if you have a large army of skeletons, it may not be noticeable if some of the skeleton instances in different locations are on the same animation frame. An army of 100 skeletons might only need to pose 10 different skeletons.
