Lightmass: multi-bounced sky lighting

10 Sky Light bounces is a little extreme and is likely where your time baking has been spent. It is non-distributable, and I believe done on a single core, and scales exponentially with each bounce you set. Sky lighting and emissives are generally not the brightest thing in your scene, so naturally their bounces are less noticeable.
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I’m sorry but number of bounces don’t effect the lighitng calculation time!!