Thanks for the additional info. Your spine-unity version already contains an important bugfix in this regard, so this is not the problem.
In general SkeletonMecanim comes with some overhead, but it should in general not be that much of a difference. Could you perhaps send us a minimal Unity project that still shows this problem? You can send it as a zip package to contact@esotericsoftware.com, shortly mentioning this forum URL so that we know the context.
General guidelines to improve performance can be found here:
Metrics view - Spine User Guide: Performance
For spine-unity, you might additionally want to check out SkeletonMecanim - Advanced - Update When Invisible in the Inspector, in order to keep off-screen updates minimal, where possible.
