Here are the Volume clouds - super lite version. This is with the directional light completely stripped out, which makes it MUCH more performant - and it's not really noticeable is it? 90% of the effect is carried by the ambient and SSS contributions. The SSS gives a general impression of the light direction, and the ambient creates the top down shading which gives the clouds their volume.
So if you require more performance, try stripping out the directional light contribution completely.