> 40 s to render at 768x1024 on a humble 1 GHz machine.

It's a matter of avoiding unnecessary "difference", "intersection" and the
like. For example, imagine this model: A large box with 100 holes. In this
case you need CSG "difference" but the rendering will be slow, unless you
split up the large box into 100 smaller boxes, each with one hole in them.
If you don't do that, meaning if you stick with one large box with 100
holes, the bounding box of each hole will expand to fill the entire box. In
other words you are rendering 101 objects on top of each other, all over the
large box. Instead of having to render only 2 objects at a time .. small box
with a hole.

This kind of bounding troubles also arise when you nest objects by using
"merge" instead of "union".. And it's dangerous to nest several layers of
"difference" of course. My rule of thumb is to avoid boolean operations when
possible. This will make even complex CSG render just as fast as a mesh -
provided you stick with old fashioned primitives, not sphere_sweeps, iso's
etc...

Regards,
Hugo

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