Comment by wilberton

In essence mega-lights is ray tracing one shadow ray per pixel, then using some temporal de-noising to try and smooth away the noise. This is in contrast to how games traditionally calculate shadows, which is by rendering a shadow map from the point of view of each shadow casting light in the scene. For a small number of lights, the shadow map approach will be considerably faster. What mega-lights enables is effectively an unlimited number of shadow casting lights for a fixed cost (because it only ever traces a fixed number of rays per pixel), but at a much higher base performance cost, and with the potential for noisy artefacts caused by the denoising.