Make sure you don’t use physical assets (ragdolls) or at least make sure they are not colliding with anything in the world. Yes, the “ragdoll” can cause performance issue when it is not active. The bodies of the physical assets follows the joints of the character when animated and is evaluated in the physics engine.
You should be able to have hundreds of skeletal meshes without performance issues. Note that skeletal mesh performance is transformation matrix multiplied by the position by each vertex multiplied by the number of influences per vertex. It is the number of vertices that decides calculation costs, not polygons. (polygons are surfaces defined between vertices) Then you also have to take into consideration if the deformation is calculated on the GPU or CPU. In UE3 it was possible to decide which, I haven’t looked into the settings for UE4.