Wasn't it LibraryMan who wrote:
>I have an object which I created two versions of, which look quite
>similar.  I had read somewhere on this group that in general, CSG
>difference takes more time to trace than union.  But when I rendered the
>two "versions" of the object in question ( a section of arcade from yet
>another Gothic cathedral scene) the version with lots of difference's 
>rendered faster than the version that was all unioning and translating.

The problem with multiple differences is that POV can only set a single
bounding slab for the whole thing. If you've got one big object with
lots of little bits cut out of it, then POV ends up having to test every
one of the CSG components for every ray. (See the "hole tutorial" at 
<http://www.econym.demon.co.uk/holetut/>). Whereas, for a union, POV can
usually create a sensible hierarchy of bounding slabs, and only tests a
small number of slabs and CSG components for each ray.

Perhaps you somehow managed to end up with sensible bounding in the
differenced object, or with very poor bounding in the unioned object. It
should be quite obvious from the stats if either of these is the case. 

-- 
Mike Williams
Gentleman of Leisure

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