Capturing the frames online without slowing the pace of the game

How to capture the background frames while a unity game is running?

I already knew

cam.Render();

Texture2D image = new Texture2D(cam.targetTexture.width, cam.targetTexture.height); image.ReadPixels(new Rect(0, 0, cam.targetTexture.width, cam.targetTexture.height), 0, 0);

and then

converting the texture into an image using EncodeToPng/JPG

But what I have seen using this extra cam.render and encoding to png and capturing the frames differently slow down the operation drastically, takes huge time to do this.

How can I directly get the textures or frames online while a game is running, maybe from the OpenGL calls which GPU are using?

Does anyone have any idea how to achieve this?