Some things you might notice with Luxrender:

Brightly lit scenes render faster than dark ones.

Lights render faster than emitters.

Emitters generally look better than lights. Sometimes you need to mix: an emitter slightly brighter with a dimmer light. The hydra render I posted earlier is three light sources - one point light, set very dim (picks up glossy highlights a bit better) and two emitters. One above and one below. Emitters create a softer shadow.

Laurie