Backwash uses tons of transparent fog effects that drift around. That's expensive, because you have to draw the scene, and then draw each fog object by blending the fog object colours with the colours of the already-drawn scene based on alpha (transparency) values of the fog objects. This permits a fog effect that looks really good and has lots of structure (it looks like actual clouds of fog drifting around), but is very rough on your graphics pipeline. It requires colour blending and a lot of redraw.

I haven't tried this out in Halo 4 yet, but from the videos, this fog seems to just change the colour value at each pixel based on the distance of the pixel from the viewer. Which from what I understand can typically be done fairly efficiently; you don't have to redraw anything because the fog effect can be calculated on the initial draw pass, and the effect is only calculated once at each point in the scene (not always the case for alpha-blended fog objects; it's a little scary when you have multiple fog objects on top of each other, as some pixels might be blended and redrawn several times).