With large meshes you’re going to see this large import time when it’s processing the data for use in UE4.
To test import length I made a sphere with 15 million polys that was around 600 mb and imported that single mesh into the engine, but it took over an hour with my machine. This may vary from machine to machine dependent upon specs.
While the engine does run smoothly with the object in my scene it will take a while to import. This is not a problem with FBX import so much as it’s a large file that has a lot of information to process.