Your scene has a nice foggy kind of effect but the clouds are not really the Fuzzy Cloud Balls that I was referring to. Also- there is very little really "volumetric" happening in your particular scene. Rendering against a relatively plain flat background IS pretty fast- but that's not what I was thinking of... However- if you WANT to use Fuzzy Materials- like Furry-Fuzzy Cloud Balls- I would suggest rendering them against a BLACK PANEL- saving them as 2D Bill Boards- and using the "Flight Simulator Clouds" approach. The 2D versions, with Alpha Channel Masks- actually work quite well. Plus you can multiply them in groups and have the "Fuzzy Volumetric Cloud Look" at hugely reduced CPU rendering time.