The distance fog uses a clipping plane, meaning anything past a certain distance is simply not drawn by the video card. r_speeds are counted by the engine. Every time it draws a polygon, it adds to the count.
You can play around with this and see if fog improves your framerate by doing the folllowing:
Make the map without fog, turn on cl_drawfps, go to a spot where the framerate is low, then go to the console and type: sky "fog 0 0 0 500" and see if the framerate goes up or down.