Lumen performance considerations

I’ve been looking around for documentation on optimizing for lumen but I haven’t really found much beyond a few tid bits.

I had some additional questions if someone can answer them:

  1. What is the impact of additional lights in a scene? Can I drop hundreds or even thousands of lights in a scene and still expect real time performance? Or is it the same as any other deferred renderer in that regard? Or worse?

  2. The documentation states: “Emissive materials propagate light through Lumen’s Final Gather with no additional performance cost. However, there’s a limit to how small and bright emissive areas can be before they begin to cause noise artifacts. This is inherently much more difficult to solve than manually placed light sources.”

Is this really no additional performance cost? If so, in a scene with potentially hundreds to thousands of light sources, would emissive materials be better than actual light sources?

  1. I understand that there is a delay in lumen updating. Does this apply equally to light sources and emissive materials?