I tried an experiment rendering a rather simple scene (a few primitives on a
plane with one light) using sse2 b3.  My machine has a P4.3GHz processor
with HT.

Threads  Time  % improvement
-----------------------------
1        72.58      0
2        63.83     12
4        64.90     11
8        59.90     17
16       54.75     25
32       50.20     31
64       48.27     33
128      49.00     32
255      47.83     34

So it seems that for a simple scene I can get a 33% speed increase by using
64 or more threads (rather than 1 thread).  Is there any reason for this,
and due to memory transfer etc will it not be as noticeable for bigger
scenes?

BTW it's also pretty cool to watch the preview when using 255 threads :-)

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