vray env fog and sample rate - speed of render

[stevesideas]

This is probably my lack of understanding when it comes to sampling etc but I have a strange phenomena. When I have the global subd's at 0.5, the max subdivs at 30 and the color threshold at 0.02 it renders a lot slower (when using vray env fog) than having these settings: global subdivs at 20, max sub divs at 20, color threshold at 0.015. Normally when I use these 2 different set ups they are as expected with regards to render time.

Can someone explain this ?

[tashko.zashev]

Hi Steve,

There should be no big difference in the render time between these setups. What is the difference in the Samplerate render element?
Is it possible to attach this file?

I suspect this is the latest V-Ray 3 SP.