Track percentage of screen filled with LineRenderer objects

Hello,

on my 2D mobile application a user can use his finger to draw white lines on a black screen. I would like to find out how much of the screen is white already, best in percentage of screen space covered. This needs to be fast on mobile devices.

For drawing I use the LineRenderer: TouchPhase.Began creates a new LineRenderer and TouchPhase.Moved adds a new Vertex to the renderer.

In order to find out how much is covered with white I currently use OnPostRender on the cam to create a texture2d and count the average color of pixels:

 using UnityEngine;
 using System.Collections;
 
 public class CountSwished : MonoBehaviour {
 
     private int targetWidth, targetHeight;
     private Texture2D tex;
 
     void Awake() {
         Camera cam = gameObject.GetComponent<Camera> ();
         targetHeight = cam.targetTexture.height;
         targetWidth = cam.targetTexture.width;
         tex = new Texture2D (targetWidth, targetHeight);
     }
 
     void OnPostRender() {
         int mipLevel = 5;
 
         tex.ReadPixels(new Rect(0, 0, targetWidth, targetHeight), 0, 0);
         tex.Apply();
 
         int count = 0;
         float avg = 0;
 
         Color[] pixels = tex.GetPixels(mipLevel);
         foreach (Color color in pixels) {
                 avg+= color.r;
                 count++;
              
         }
         float swished = avg / count;
         Debug.Log(swished * 100);
     }
 
 }

This outputs percentage of pixels covered in white to console. However it is too slow and drops the framerate from 60fps to something like 15fps. Is there a better way to do this? Going pixel by pixel does not seem to be a good idea - maybe there is some totally different way?

My cameras have static positions. Drawing on a surface that is already white should not increase the computed percentage covered.

Thanks a lot, Martin