It's important to note that I am building for Windows standalone 64bit which is different than WebGL platform that OP mentioned.
To your question:
If I comment the line
AssetDatabase.FindAssets("t:Prefab");
then the delay is removed entirely.
I have decided to check what is happening if I do let it stay and check the rest of the process and I have found that
most of the hang up is coming from around
AssetDatabase.LoadAssetAtPath<GameObject>(assetPath)
and
SpineEditorUtilities.CleanupSpinePrefabMesh(prefabGameObject)
.
I am suspecting those areas because the current test I ran the build finished completely in 2:30min but 1:36min of it was up until the point above the lines listed above (after the entire foreach loop has ended and it reached
AssetDatabase.StopAssetEditing();
).
Be aware that it's not always going to take 2:30min for a build, yesterday it took me more than 11min. The times are completely random it seems.