Overlapping volumes in Cycles create huge render time. Is there a solution?

[silex (Jakub Jaszewski)]

So I’ve been thinking how to solve this problem.

I wonder if there might be a way to use Volume cube to recreate volumes from shader editor.
I have a personal interest as I’m trying to mix OpenVDB from Mantaflow with some procedural volumes generated with very similar method to one described by @SterlingRoth above.

First step would be recreating procedural volume shader inside Geometry Nodes. However this is not as straightforward as plugging Gradient Texture to Empty mapping, throwing some noise on top and calling it a day - like you would do in Shader Editor.
For a start I picked a method described by @Leonard_Siebeneicher in SDF thread:

SDF, Volumes and Force Fields

You get an SDF if you subtract an offset from a distance field. A few basic SDF’s An SDF of a sphere does look like that … [sphere] Just plug the result to a Volume Cube node, and you will get a volume modell. We could get a cylinder SDF from a sphere SDF. Just project input coordinates 3D to a 2D plane. [cylinder] SDF Booleans Booleans could be realized with math nodes, using “Minimum” and “Maximum”. [SDF_Booleans] Blender does offer “Smooth Minimum” and “Smooth Maximum”. They do ena…

Here is first naive attempt to see how recreating shader based volume density looks in Geo Node SDFs:

Shaded Volume (desired effect):

Geo Nodes:

Geometry Nodes are set on top of 1 vertex mesh object with the origin at 0,0,0 parented to an Empty which is referenced in GN tree as WORLD ORIGIN. This construction can be simplified, but it’s convenient for my use case.
But so far the outcome is completely different.

If shader recreation pan out - next step would be to convert both volumes to mesh, then joining them and finally converting back to volume.