Additionally, if you're using Spine-Unity, there are a number of additional options to improve performance. But it highly depends on what your animations are like.
For example, if each of your skeletons only use one Material and don't use separators, you can check the "Single submesh" checkbox in the Advanced... box in the inspector. But depending on your setup, this may not have much of an impact in performance.
If you can show a screenshot (of the profiler window) of what specific methods in the Spine runtime you are hitting hard, we may offer some more concrete recommendations.
