Optimize code changing transparency of large number of Objects

I have a wall made up of a about two thousand cubes. Each cube has a material that changes transparency with time. I change the alpha value of each material based on a randomized sine function. The problem is that when the player gets far away from the wall, the frame rate tanks. How can I optimize the code below to improve my FPS?

       // Use this for initialization
     void Start () {
         rend = GetComponent<Renderer>();
 
         if (type == WallType.blue)
             rend.material = blue;
         else if (type == WallType.red)
             rend.material = red;
 
         color = rend.material.color;
         height = Random.Range(.1f, 1f);
         stretch = Random.Range(1f, 3f);
         offset = Random.Range(.1f, 6.28f);
         origZ = transform.localPosition.z;
     }
     
     // Update is called once per frame
     void Update () {
 
         alpha = height * Mathf.Sin(stretch * (Time.time + offset));
         if (alpha > .1)
             color.a = alpha;
 
         rend.material.color = color;
         
     }