We have not yet encountered excessively long import times, on the other hand, our test projects don't usually contain many prefabs.
What exactly do you mean by "mecanim files"? Does this only occur when upgrading from an older Unity version, or also upon every Reimport All
? If it happens only once, I'm afraid it will not be important enough of a task to be prioritized in the near future.
You could try if it's related to Spine or not by removing the Spine components from your prefabs, or duplicate a single test asset with and without Spine components many times so that it takes enough time to compare both variants.