Some of the images on the Luxrender site took 20-90 hours to render. CPU unbiased rendering is not a fast rendering solution if someone should think that. But as Bill said a GPU can be 10 - 50 times faster but is still limited to the ram on the gfx board.
Fireflies are a side effect of unbiased rendering. Im guessing that Luxrender have a MLT filter or some other options to prevent that from happening. Octane also have those fireflies now and then, but will soon have a filter for that.