High GPU cost of a color parameter

I have a very simple shader that only reads a texture and multiplies it with a color. I've given it below. In my scene, I'm using it to draw a few 2D objects, filling the screen maybe two times over. The problem is that, on my iPhone 4 at least, the GPU consumption is over 30ms/frame. However, if I remove the multiplication by the _Color parameter at the end, the GPU consumption drops to 11ms or so.

This seems quite weird to me. How does a multiplying by a simple color variable eat so much GPU? I've also tested replacing the _Color variable by a simple "float(0.5, 0.5, 0.5, 0.5)": the GPU time remained around 11ms.

Here is my shader:

 Shader "Custom/MultiResSpriteShader" {
     Properties {
         _Color ("Main Color", Color) = (1, 1, 1, 1)
         _MainTex ("Base (RGB)", 2D) = "white" {}
     }
     SubShader {
         Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}
         ZWrite Off Lighting Off Cull Off Fog { Mode Off } Blend SrcAlpha OneMinusSrcAlpha
         LOD 110
         Pass {
             CGPROGRAM
             #pragma vertex vert_img
             #pragma fragment frag
             #pragma fragmentoption ARB_precision_hint_fastest
 
             #include "UnityCG.cginc"
 
             uniform float _Color;
             uniform sampler2D _MainTex;
 
             float4 frag(v2f_img i) : COLOR {
                 return tex2D(_MainTex, i.uv) * _Color;
             }
             ENDCG
         }
     }
 }

Any thoughts would be appreciated.