UE4 has to process the mesh when it imports, which takes a lot of extra processing time. If you don’t change any of the default import settings it’s even worse because it will try to generate lightmap UV’s which adds even more processing.
It’s likely though that you don’t need a mesh that high poly, unless it’s like a giant tree like Avatar then most likely you’re overdoing it. There’s tree systems like Speedtree that optimize stuff like that for games.