oh!  now i get the weird fireflies and the long time.  you were making glossy materials,   which reflect and probably have ideally sharp highlights by default.

i'm thinking this type of testing doesn't work well for Luxrender.  empty world renders, that is.    since the red is getting some blue (according to my eyedropper extension) and the glossy seems to be picking up random highlights, it makes me think it might calculate based on the notion of some sort of sky surrounding the scene if there's nothing in the way.  just a thought.

edited to add: maybe Cornell box renders would make more sense?