More about the above comparison. There are two reasons why this rendered faster using the built-in soft shadows. Firstly I only used 9 shadow samples, rather than 21 as in the multiple-sun setup. The built-in soft shadows are able to randomise the sample positions which means you don't need as many samples or suns to avoid steps in the shadow boundaries, so 9 samples are adequate for this scene. Second, the built-in soft shadows use less shadow samples when calculating the light falling on the clouds and atmosphere, because the quality reduction there is less noticeable. That optimisation is not possible if you use the multiple sun trick.
Because of the optimisations in soft shadows on the clouds, I expect that if I had enabled ray-traced shadows in the clouds the speed difference would be much greater.
Matt
