I assume you are using skeletons in JSON format. Unfortunately, the JSON deserializer produces a lot of garbage, which can be resolved by exporting the skeleton in binary format.

When profiling your game run in the Editor (and not built to a target platform and attaching the profiler there), I just noticed that JSON deserialization will currently produce double the GC overhead, by checking for version compatibility in the Editor and performing an additional deserialization step. I will fix this problem, as it nearly doubles the displayed GC statistics in the Editor compared to when it its built to a target platform.

New unitypackages fixing the unnecessary additional JSON deserialization overhead in the Editor have just been uploaded.
You can get them here as usual:
Spine Unity Download

This issue has been tracked under this issue ticket:
https://github.com/EsotericSoftware/spine-runtimes/issues/1608

Yes, we would recommend re-exporting the projects in binary format to lower the memory footprint and speed up loading.

If you have many projects, we suggest automating exporting your project files:
Export - Spine User Guide: Command line
For example, here is a script we use to export all the Spine example projects and to create texture atlases:
spine-runtimes/export.sh at 3.8