Postfx is basically a 2D operation, they are normally done on the entire screen using just two triangles to cover it, and the operations they do aren't putting any significant load on modern GPUs (just some number crunching) - even bloom and tonemapping which tends to render many 2D images (1x1 size, ½x½ size and so on, sometimes all the way down to 1x1 pixel) and them combining them to an output image, all this is kids play for GPUs :D