title: Unity ReadPixels() is slowing game down
body: <p>I am using the following to take the content of a camera and write it to a material. This works fine with 1 camera, but when I attach it to 4 cameras and write to 4 materials it drops the FPS to 12 - 13 FPS. Is there something I can do to optimize this?</p>
  
  <pre><code>using UnityEngine;
  using System.Collections;
  
  public class RenderToTexture : MonoBehaviour {
  
      public Material mat;
      [HideInInspector]
      public Texture2D renderedTexture;
  
      void Awake () {
          renderedTexture = new Texture2D(Screen.width, Screen.height);
  
          mat.mainTexture = renderedTexture;
      }
  
      void OnPostRender(){
          renderedTexture.ReadPixels(new Rect(0, 0, Screen.width, Screen.width), 0, 0);
  
          renderedTexture.Apply();
      }
  }
  </code></pre>