I have noticed some kind of similar media-rendering behavior, especially with
scattering media (but also with emission + absorption.)

I'm running V3.8.0 beta 1  (Windows), on a core I7 8-core/16-thread machine.

I currently use the default 'render block size' of 32X32 pixels and no 'mosaic
preview'. The last few 'render blocks' that get processed in my images (which
are in areas where there is media) seem to cause my machine to work *extremely*
hard. I have noticed this in two different ways: My machine's cooling-fan speed
greatly increases; and the Windows 'Task Manager' app shows that those last few
blocks cause the processor to suddenly jump to nearly 100% use.

On my machine, there does not seem to be an actual slowdown in the render-- just
some very hard processor work behind-the-scenes.

This is my *guess* as to what is going on (simply from a user's standpoint, and
not a programmer's):
Due to symmetric multi-processing, it would seem that each 'render block' in the
image is being processed by a particular core/thread(?)-- or, the blocks for
most of the render are being shared among those. But for the very last block--
or last few blocks-- the SMP process is reducing the number of threads(?) to
process those... in other words, maybe the last block is using only one core,
and trying to make it do *all* of the processing... resulting in a slowdown (or
overheating of the processor, in my case.)

Again, this is purely guesswork. I don't know the inner workings of processors
or POV-ray's SMP stuff ;-)

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