I could be wrong, but I think that a lot of the older variations of volumetric lighting are of the simple type that just voxelize a cheap fog gradient. They aren’t actually using textures to produce the fogginess. Therefore it would be a lot cheaper on draw and wouldn’t be that big of a hit to frame time.
But if you want fancy “placed” patches of clouds like
except with way higher fidelity in the “cloudiness” like this: and you want them to be dynamic with things like extinction colors, shadows and so on; you’re going to massively increase the calculations needed.
EDIT: Oh and I should say that the moral of the story that there is quite a difference between volumetric fog and volumetric lighting. Fog is cheap, therefore any volumetrics you apply to it are likely going to be cheap as well. However, using volumetric lighting on complex clouds of voxels, with actual textures, is exponentially more expensive.
EDIT 2: Here’s a pretty good PPT that breaks down some different methods and I guess I was sort of right about how a lot of the older games were approaching it:2014/08/bwronski_volumetric_fog_siggraph2014.pdf
as well as https://developer.nvidia.com/sites/default/files/akamai/gameworks/downloads/papers/NVVL/Fast_Flexible_Physically-Based_Volumetric_Light_Scattering.pdf. In the Nvidia one, you can see some of the render times for it; toward the end of the slides. They were in the 1-2ms range. Remember that 60fps is 16.67ms per frame. That would mean that the lighting they were using was taking up ~12% of the frame time, on a 60fps budget. It’s worth mentioning that it also looks fairly basic and looks like it’s just glorified height fog with shadows in it. So if it was eating up 1-2ms for just that, picture how much frame time would be eaten up with true volumetric voxel clouds that have all the bells and whistles in them…