I have an old notebook which we use for test pursposes and it seems that visibility culling is not working there…
If I move the camera away from everything - I have the same fps as when I look directly at a group of objects in the scene. So, I suppose - the whole scene is being rendered. Might this be true?
Are many/all of your objects animated? The SkinnedMeshRenderer has an option called Update When Offscreen that keeps the animation cycling while the object is not visible. If you are using animated objects and updating them offscreen, this could account for a lot of CPU time and might explain why the framerate doesn’t change much.