Simple experiment:
-Open editor
-Place a default player soldier
-Preview mission
-Result: Extremely smooth framerate
-Go back to editor
-Place 20 soldier squads somewhere on the island out of the line of sight of the player soldier
-Preview mission
Result: Framerate now feels a lot less smooth and more stuttery while exactly same amount of geometry and textures is being drawn by GPU. The other entities are out of the line of sight, yet they cause significant computational overhead.