Scratch Card Is not smooth using SetPixel()

Hi,

I want to implement a scratcher where user can scratch the foil to reveal the content on card. For the following I used unity SetPixel(). But it's not smooth when user reveal vigorously.

My code is as follow :

 public class Scratcher : MonoBehaviour
 {
         public UITexture mTex;
 
         private Texture2D texture;
         private float erasedPixels;
 
         private int textureWidth = 28;
         private int textureHieght = 28;
 
         void  Start ()
         {
                 // Create a new texture and assign it to the renderer's material
                 texture = new Texture2D (textureWidth, textureHieght, TextureFormat.RGB24, true);
 
                 GetComponent<UITexture> ().material.SetTexture ("_SliceGuide", texture);
 
                 // Fill the texture with white (you could also paint it black, then draw with white)
                 for (int y = 0; y < texture.height; ++y) {
                         for (int x = 0; x < texture.width; ++x) {
                                 texture.SetPixel (x, y, Color.white);
                         }
                 }
                 texture.Apply ();
         }
 
         void Update ()
         {
                 if (Input.GetMouseButton (0) && UICamera.hoveredObject != null && UICamera.hoveredObject.tag == "ScratchFoil") {
 
                         Vector3 localPos = transform.InverseTransformPoint (UICamera.lastHit.point);
                         Vector2 uv;
 
                         uv.x = localPos.x / (float)mTex.width;
                         uv.y = localPos.y / (float)mTex.height;
 
                         Texture2D tex = texture;
 
                         Vector2 pixelUV = uv;
 
                         pixelUV.x *= tex.width;
                         pixelUV.y *= tex.height;
 
                         // add black spot, which is then transparent in the shader
                         tex.SetPixel ((int)pixelUV.x, (int)pixelUV.y, Color.black);
                         tex.Apply ();
                 }
         }
 }

Now I applied this script on my NGUI UITexture which contains a material with a shader used to clip the current texture RGB value from _MainTex.

Shader As follow :

  Shader "mShaders/ScratchEffect" {
     Properties {
       _MainTex ("Base (RGB), Alpha (A)", 2D) = "black" {}
       _SliceGuide ("Slice Guide (RGB)", 2D) = "white" {}
       _SliceAmount ("Slice Amount", Range(0.0, 1.0)) = 0.9
     }
 
     SubShader {
       Tags {"Queue" = "Transparent" "IgnoreProjector"="True" "RenderType" = "Transparent" }
       Cull Off
 
       CGPROGRAM
       #pragma surface surf Lambert alpha
       struct Input {
           float2 uv_MainTex;
           float2 uv_SliceGuide;
           float _SliceAmount;
       };
 
       sampler2D _MainTex;
       sampler2D _SliceGuide;
       float _SliceAmount;
 
       void surf (Input IN, inout SurfaceOutput o) {
           clip(tex2D (_SliceGuide, IN.uv_SliceGuide).rgb - _SliceAmount);
           o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb;
           o.Emission = tex2D (_MainTex, IN.uv_MainTex).rgb;
           o.Alpha = tex2D (_MainTex, IN.uv_MainTex).a;
       }
       ENDCG
 
     }
     Fallback "Diffuse"
 
   }

Everything working fine to it just this SetPixel() is sucking a lot of frame per second and not giving smooth scratch effect.

Anybody help me out with this trouble.

Thanks in Advance.