Vray mesh lights and objects with VrayLightMtl take twice as long to render

[Alex_M]

I'm working on something here that requires irregular light shapes so I chose to use either mesh lights or VrayLightMtl with the option "Direct illumination" turned on. The problem is, these two approaches are quite slow to render compared to other types of lights (plane, sphere...) and there's a lot of noise that is hard to clear. Is this normal? Any way to speed it up?

You can find below a simple scene for testing. The mesh lights have exactly the same shape as the ordinary vray plane light objects but they take twice as slow to render compared to them (and even worse in more complex scenes).

[svetlozar.draganov]

This is actually a normal behavior. Standard V-Ray lights allows using optimization techniques which produce faster and smoother result.
Custom geometry lights like V-Ray Light with Direct Illumination or V-Ray Mesh Light cannot be optimized that well and they usually generate more noise and requires more render time even if its shape is the same as the Standard lights. Our recommendation is to only use V-Ray Lights and Mesh Lights when is absolutely necessary.

To get clear result you may increase the Minimum Shading Rate parameter, speeding up the rendering is possible by reducing MSR and IS settings but it will generate even more noise.

[Alex_M]

Thanks for the thorough explanation. I'll definitely have this in mind and use mesh lights when standard Vray lights is not a feasible option.