YafaRay UV Cube XML Code

[micheus]

but from what I have read on the web in the past, Luxrender is much slower than YafaRay???

Probably yes. Again, time is a relative question.
What kind of result is enough for us?
I think we only can make a best comparison between those that use the same "technology". (see Spectral rendering)

I found a comment that can explain that impression:
... everything else Lux is spectral. All things being equal, it will always be slower than a simple RGB renderer. (ref. post)

and other about LuxRender vs Cycles:
LuxRender focuses on the physically-based aspect of rendering, while Cycles focuses more on the classical "production style" rendering. Thus it's harder to cheat in LuxRender than in Cycles, for example having a shadowless object. On the other hand you can use measured data and such for materials and good results with relatively little tweaking in LuxRender. (ref. post)

even POV-Ray - supposed to be fast - can spend a lot of time to render a scene if we want produce a photo-realistic image. Take a look at this image on gallery. It took 154 hours to be ready.
* It probably would be a little small nowadays in the new CPUs and using POV-Ray 3.7 that supports many cores.