Thick (highly opaque) Smoke renders faster than thin (highly transparent) Smoke because the lighting calculations for voxels completely hidden behind other, opaque voxels, can be skipped entirely.
If your Smoke is dense, you can speed up rendering further try enabling the Use Probabilistic Shading option in the Environment and Effects window to see if that gives you a boost in render speeds.
If your Smoke is highly transparent, Probabilistic Volumetrics won't make much of a difference, and in some cases may even slow down your renders.
If you are using V-Ray's Progressive Sampler, Probabilistic Volumetrics should improve performance.
If V-Ray's Global Illumination is enabled, the number of GI Samples will heavily affect the render times. Both high and low values will increase the render time - you should aim to find a good value somewhere in the middle.
