When to use "Use interpolation" in V-ray material? What does it do?

Use "Use interpolation" option in Vray material to speed up rendering of glossy reflections or refractions/translucency. Interpolation option should be used ONLY in stills! Do not use it in animations, because it causes flickering.

It allows you to set Hsph subdivs and Interp samples separately from Global subdivs. This is usually done to smooth out a material that otherwise looks too grainy and to speed up rendering of material that is otherwise too slow.

Always test it before rendering the final, because with wrong settings it might look awful.

How to use it:

1. Material editor-->VrayMtl-->Reflection or Refraction-->Tick "Use interpolation"
2. Material editor-->VrayMtl-->Reflection or Refraction interpolation tab-->Tweak the settings and test (Compare with Global settings - Irradiance settings)