VRayVolumeGrid slowing down render

[mark_aldred]

I have a VRayVolumeGrid that renders lightning fast if only it is visible. But if I introduce a PhoenixFDFire into the render, it slows down. The two objects are not intersecting. They aren't even overlapping in the render. The PhoenixFDFire renders fast no matter if the VRayVolumeGrid is visible or not.

What could be causing this and how can I fix it?

edit: there's only one light in the scene and it's illuminating only the VRayVolumeGrid. no shadows.

[mark_aldred]

Found the cause - it was the fire lights in the PhoenixFDFire. If I turn the off it's back up to speed.