There is no one definitive thread, its more like the death of a thousand cuts, try using the search for cycles and luxrender.
Its impressive that you got lux and cycles working so easily being new to blender
The answer is simple, luxrender is as fast as it can be for doing what it does, its open source so if anyone has a faster way of doing things they can present it.
Lux physically simulates light flow on x86 general cpus and scales in a linear fashion to any number of cpus.
One example of why your metric is invalid is that you compare lux with cycles on a single PC, as I implied above.
You used your PC with its gpu, lux is not limited to one PC so you can't really say how fast it is. You could use the Amazon cloud with lux and it would render millions of times faster than Cycles.
Also your plane and cube test scene has no meaning, try something more complex with physical materials like glass.
Anyway, I could talk about this for hours but you are missing the point so I will restate it...
Its not about speed its about appearance.
If you are happy with the results just use Cycles.