You can try grouping them up in maya first as a test. I’d say beyond that, it would have to be a memory issue in which case it would be time to start going after the poly count and texture size. I’d also suggest doing LOD work so that the engine can swap in lower quality models for distant assets.
You might want to look at the statistics for your scene as well. You can do that in the editor by clicking the down arrow next to the Build button and in the drop down menu clicking “Scene Stats”. should give you an idea just how much memory you’re taking up.