So I take it Poser13 doesn't render faster after all, it just has lower quality presets resulting in shorter render times?

If you set pixel samples to 2 it will be even faster lol.

i.e. if the pixel samples, bucket size and adaptive sampling threshold etc. settings are exactly the same the render time difference is what exactly?

I might have to fire up Poser13 in a virtual machine to see for myself.