Fresh from the tome of POV-Ray Voodoo:

Beware when mixing media with complex textures using texture_map: It will slow
down rendering in an unexpected fashion!

I just found in the POV 3.7 code that media is computed *per component* in such
a case; so if for instance you try to average three textures using the "average
texture_map" construct, it will result in media effects being computed *thrice*
for any ray travelling towards that surface.

(Layered textures are safe in this respect, and so are all patterns that
strictly choose between one of multiple textures, like checker or hexagonal.)

Don't know if it has already been this way in 3.6, but in any case this asks for
a change...

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