I think skeletal animations of those characters are cpu intensive. Also if they have collisions with each other, that can drop fps greatly. You should also try crowd control AI, it works better in crowds. Also if they affect navigation, it will be updated for every single of them.
Check materials they have and shadows, make them not cast shadows and use unlit materials to see if that is any real impact.
But i also found out that 40-50 skeletal mesh actors is about limit for unreal.