So I am trying to learn how to do screen effects in Unity. The problem is that ANY effect is slow on mobile. At first I thought it was the complexity of my shader, so I tried it with a much simpler shader:

Shader "Hidden/NewImageEffectShader"
{
	Properties
	{
		_MainTex ("Texture", 2D) = "white" {}
		_Amount ("Effect Amount", Range (0, 1)) = 0
	}
	SubShader
	{
		// No culling or depth
		Cull Off ZWrite Off ZTest Always

		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			
			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
			};

			struct v2f
			{
				float2 uv : TEXCOORD0;
				float4 vertex : SV_POSITION;
			};

			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = UnityObjectToClipPos(v.vertex);
				o.uv = v.uv;
				return o;
			}
			
			sampler2D _MainTex;
			uniform float _Amount;

			fixed4 frag (v2f i) : SV_Target
			{
				fixed4 col = tex2D(_MainTex, i.uv);
				//Make it white
				col = col + _Amount;
				return col;
			}
			ENDCG
		}
	}
}

It's a very basic adaption of the shader Unity provides as default. All it does is add 1 to the color

The script to run the shader looks like this:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class TurnThingsWhite : MonoBehaviour {
	
	public float _Amount;
	public Material material;

	float Timer = 2f;

	void Update(){
		if (Timer > 0.1f) {
			Timer -= 1f * Time.deltaTime;
		} else {
			Timer = 2f;
			_Amount = ((_Amount * -1f) + 1f);//toggle binary
		}
	}

	// Postprocess the image
	void OnRenderImage (RenderTexture source, RenderTexture destination){
		material.SetFloat("_Amount", _Amount);
		Graphics.Blit (source, destination, material);
	}
}

On Desktop this has almost no impact on performance, on mobile this is dropping even new mobiles down to 30fps-40fps. With nothing else in the scene except a 3D text object to see the fps.

Is this a problem with my script? Is there some other way mobile screen shaders should be made?

5 minutes ago, Seabolt said:

OnRenderImage though, I manually did Graphics.SetRenderTarget,

This looks like my problem. It's running above 60fps now. Strange that this one thing had such a huge difference between desktop and mobile. Even works with my shader on old mobiles.