which is the most efficient additive transparent shader?

I can't seem to get decent frame rates with any of them on a couple of large objects in the scene.

I'll assume you just need a texture. Let me know if you need further functionality. Also, please tell us what the target platform is; this one would be ideal for iOS, but a desktop variant could run faster if alpha testing were involved. And if this is for iOS, and your specific target platform is iPad or anything with a retina display, then you might just be out of luck, because the GPUs in those devices don't have the fill rate necessary to run shaders that require overdraw, with a reasonable framerate.

Shader "Additive Texture" {

Properties {
    _MainTex ("Texture", 2D) = ""
}

SubShader {
    Tags {Queue = Transparent}
    Blend One One
    ZWrite Off
    Pass {
        SetTexture[_MainTex]
    } 
}

}