If the Dist. end works like it does in some other render apps I have (they call that 'far attenuation'); setting a value for it can signifigantly reduce render times by effectively limitting the number of intensity fall-off calculations.
For a linear fall-off, zero can be reached mathematically and the light will 'end' at some point; but...
For a quadratic fall-off formula, if there is no 'cut-off' distance supplied, the fall-off calculations will continue on into whatever passes for infinity in Poser; as the light's intensity will not ever reach zero mathematically.
Scenes with more area that is not confined by geometry (to stop the light rays); are more succeptible to this phenomenon.
I do not know if this is true in this case, as I do not render w/ Poser; but it might be worth looking into with some simple controlled timing tests.

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