The FBX import process can take a while depending on a number of factors.
complexity of the static/skeletal mesh
whether it is triangulated or not
If the mesh has the import option for Combined enabled, this can take a while for a lot of meshes because the FBX will be loaded, saved one part as a usable editor asset then save the next piece to it.
Your systems spec play a part in this as well.
To test this, disable Combine Meshes on the import options window and import. You should see that it imports the mesh more quickly since it doesn’t have to combine and resave the uasset every time it adds another piece.
As an alternative you can use the Full Scene Importer (new to 4.12) to import larger scenes that brings the meshes in as separate actors and can combine them into a single Blueprint actor or place it in the scene. You can do this by going to File > Import into Scene to see how it works.