The shutter speed is the same of a physical camera. Setting it at 1/60" will bring half the light to the film plane than 1/30", which will result in a darker image. Because Lux renders the whole image right away, instead of using line-scanning like 3Delight, you can see the result after a few seconds, and you can adjust the exposure on the fly, while the image renders. The first pass is very grainy and successive passes reduce the noise until it goes away. 

The render time is, if you want to make a parallel with real life, the time that it takes to develop the image on paper after it's taken. The exposure settings mimic exactly the exposure settings of a physical camera. That is one of the great features of Lux which is possible because the renderer does not render in RGB, but instead it calculates the light spectrum. So, once you set the exposure the time spent doesn't change the darkness or lightness of the image. It will change the amount of noise.

Going back to the image that started this thread, that is the reason why I pointed out the overexposure. The image does not show noise as in grain, it breaks up because of the overblown highlights. That will not change with render time, it's a matter of adjusting how much light hits the film plane.

All the best.

Paolo

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