Insane render times of objects inside Phoenix FD grid

[anowia]

I'm rendering a car with Phoenix FD dust behind it, and having Phoenix FD in the scene increases the render time of the car itself more than tenfold (compared to if I set the PhoenixFD object to "not renderable"). This happens even if I do a region render on just a small area of the car (like the one marked with white), which should not be affected by the dust at all.
The car is inside the grid. If place it outside the grid, the render times are normal again.

Increasing the step % in the rendering tab reduces the render time on the car, proportional to how much I increase it.
Disabling "Cast shadows" seems to almost completely solve the problem, so it seems to have to do with the way shadows work on objects inside the grid. But there is no shadow on those parts of the car at all, the result with and without PhoenixFD enabled is completely identical.

Is there a way to optimize or get around this? I'd rather not have to deal with such insane render times in an already very heavy scene.
I'm using Vray 3.2 and Phoenix FD 2.2

[Svetlin.Nikolov]

Hello,

Since you are using V-Ray 3.2, I'd suggest a test - hide the Phoenix grid and substitute it with a VRayVolumeGrid - it's the Phoenix shader, but a much more recent version with various optimizations. You have to load the aur caches and replicate the render settings (in future official versions we have a preset system that can transfer settings between volume grids and phoenix objects). See if that makes an acceptable difference.

Also, what is the resolution of the grid? Are you using GI? Do you have MB on the smoke as well?

[anowia]

That made a huge difference! Now I can even crank the samples and still have a quarter of the previous render time. I'm guessing future versions are going to feature this updated shader. Thanks!
I did have MB on the smoke, no GI. The resolution was about 60m.

[Svetlin.Nikolov]

Nice, glad it works better! If you still find it slow, you could drop the scene to support so we can check if there isn't something special in it slowing it down. Otherwise, I recommend to make sure the Phoenix light cache is on, and you can experiment with increasing the "Subdiv. reduction" parameter too (it's called 'light cache speedup' in the volume grid).