Low fps with day/Night cycle

I have a simple scene in my game, a village, two terrains. My fps drops dramatically when I turn on my day/night cycle which consists of two indirect lights with a simple script that makes them rotate around the center, using

 void Update () {
     transform.RotateAround (Vector3.zero, Vector3.right, speed * Time.deltaTime);
     transform.LookAt (Vector3.zero);
 }

Is there any way around this or I can't have a day/night cycle in my project?

My email - mundiproject@hotmail.com