ReadPixels Performance

I have a 3d scene with a special "mouse picker" camera that uses a solid color replacement shader and renders to a RenderTexture. Each object has a unique color that allows me to identify what object the mouse is currently under. The camera uses a custom projection matrix that allows us to use an extremely small (2x2) render texture which is only the few pixels under the mouse pointer.

All this works great. What doesn't work great is the performance of the call to Texture2D.GetPixels. On modern hardware the game doesn't even skip a beat. However, on older hardware, that one function call is extremely expensive. It also seems to be extremely dependent on the size of the screen buffer. The higher the resolution, the worse the performance hit from this call.

Here are some specifics:

• The camera is using forward rendering

• The render texture is created using:

  new RenderTexture( (int)( 2 * AspectRatio ), 2, 16, RenderTextureFormat.Default, RenderTextureReadWrite.Default);

• useMipMap is set to false

• ReadPixels is called in the OnPostRender for the picker camera:

  readTexture.ReadPixels(new Rect(pickPosition.x, pickPosition.y, 1, 1), 0, 0, false); readTexture.Apply();

• I tried delaying the ReadPixels call for a few frames and it makes no difference

Is there something I am doing wrong? Is there a way to defer the ReadPixels call so it doesn't freeze up the main thread? Is there a completely different way to do this? Any help would be appreciated!