Maya Fluids Fog

I tried to use the Maya fluid system to create a fog for the clouds, after a failed attempt trying to use sprites. However this method of creating the fog increases the render times to an unacceptable amount. However I an aware of another technique used by Alex Alvarez in several of his images (Including "Dirge" which was the basis for this project initially).
However here he explains his use for creating some background fog in his image "Gathering"
I hope to utilize the same technique as well as some expressions to get an animated fog that renders quickly.