Overlay Effect Shader - Very slow in mobile device

Hey I have written a CG shader for photoshop overlay effect for 3 textures(one base and other two are overlayed on it), it works fine on desktop but in ipad1 and iphone4 the fps is going down to 9-10 fps.. Is there a workaround.. Here is the code-

 Shader "Custom/Overlay" {
     Properties {
         _TexA ("TexA", 2D) = "white" {}
         _TexB ("TexB", 2D) = "white" {}
         _TexC ("TexC", 2D) = "white" {}
     }
     
     SubShader {
         Tags { "RenderType"="Opaque" }
         Pass {
         Lighting Off Fog { Mode Off }
         CGPROGRAM
         
         #include "UnityCG.cginc"
         #pragma vertex vert_img
         #pragma fragment frag
         #pragma glsl_no_auto_normalization
         #pragma fragmentoption ARB_precision_hint_fastest
         sampler2D _TexA, _TexB,_TexC;
 
             fixed4 frag( v2f_img i ) : COLOR {
                 fixed4 a = tex2D(_TexA, i.uv);
                 fixed4 b = tex2D(_TexB, i.uv);
                 fixed4 c = tex2D(_TexC, i.uv);
 
                 fixed4 r = fixed4(0,0,0,1);
                 if (a.r > 0.5) { r.r = 1-(1-2*(a.r-0.5))*(1-b.r); }
                 else { r.r = (2*a.r)*b.r; }
                 if (a.g > 0.5) { r.g = 1-(1-2*(a.g-0.5))*(1-b.g); }
                 else { r.g = (2*a.g)*b.g; }
                 if (a.b > 0.5) { r.b = 1-(1-2*(a.b-0.5))*(1-b.b); }
                 else { r.b = (2*a.b)*b.b; }
                 r = lerp(a,r,b.a);
                 r = lerp(b,r,a.a);
                 fixed4 x = fixed4(0,0,0,1);
                 if (r.r > 0.5) { x.r = 1-(1-2*(r.r-0.5))*(1-c.r); }
                 else { x.r = (2*r.r)*c.r; }
                 if (r.g > 0.5) { x.g = 1-(1-2*(r.g-0.5))*(1-c.g); }
                 else { x.g = (2*r.g)*c.g; }
                 if (x.b > 0.5) { x.b = 1-(1-2*(r.b-0.5))*(1-c.b); }
                 else { x.b = (2*r.b)*c.b; }
                 x = lerp(r,x,c.a);
                 x = lerp(c,x,r.a);
                 return x;
             }
             ENDCG
         }
     }
 }

Your answers are very much appreciated.. Thanx