Vray Volume Grid & Vrayenv fog are very slow together but not individually

[stevesideas]

Is there a reason why if I turn on one of the above effects individually they render fairly quickly but having them on together suddenly slows the render down to maybe 1/6 - 1/8 of the speed?

[hermit.crab]

This is normal and is caused by the vast amount of internally created polygons when in Volumetric Geometry mode. To optimize render times you may use another VolumeGrid (without a cache) for the fog instead.

To do that:
1. Create a new VRayVolumeGrid
2. When asked for a cache, click Cancel - this will create an empty VolumegGrid node
3. In its Volumetric Options, set the Smoke Opacity to be based on Smoke
4. Adjust the Color of the fog through the Smoke Color. Adjust the density of the fog through the Opacity diagram (i.e. setting the Y of the first point to 0.0001 will produce a very thin fog)
5. Control the size of the "gizmo" through the Grid> XYZ parameters of the VolumeGrid itself.