In Indigo Manual we fin this quote regarding Layers and optimization:
Despite this, Layers are a very useful way to tweak multiple lights with different light strength, and how they behave against one another, we can say it is a lesser evil, to which one, after some tweaking might revert, if not by the way Indigo handles lighting strength.If there are many light layers used, it will consume a lot of memory. If this exhausts all physical memory the rendering will become extremely slow due to disk access.
Say you have a candle lighting part of your scene and a light bulb for the overall scene, Indigo rightfully interpret the differences of scale between both, but it goes further, it assumes that the candle doesn't have much relevance to the scene by its sheer amount of power, the thing is, the candle will then add a lot of the noise to the scene, whereas if both share the same strength it wouldn't negatively interfere that much. One could improve the render scene times by equalizing both light strengths, the only downfall is that we would then only control the relative light intensity through Layers alone.
IMO simplicity is key, and having all lights equalized regarding their cpu importance by default, would improve the vast majority of the cases, 2 lights 50% each,4 25% and so on.
Presently to achieve it I use Light normalization set to a given Luminous Flux, and multiple layers to control the relative strength. Here you can see how few discrepancy between the major light, in the left one doesn't have much difference, not at least when compared with the weaker light and the overall Picture.