The fog has nothing to do with view distance. The fog is just there to make objects (buildings, terrain, soldiers, etc) blend with the skydome texture.

In fact if you removed fog alltogether I think you'd find it to be faster, since you no longer has to calculate the fog color on top of whatever has been rendered beneath. Well relativly speaking, fog isnt all that resource heavy

Dawdler you use fog to mask the Max view distance. You could set the view distance to 5metters and the fog to 4 and have a sandstorm/Snowstorm map with perfect performance .

General Un-Co,
                         Signing off