POV-Ray : Documentation : Performance considerations
  POV-Ray 3.6 Documentation Online View Normals and Radiosity   1.3.8 Making Animations Performance considerations

Radiosity can be very slow. To some extend this is the price to pay for realistic lighting, but there are a lot of things that can be done to improve speed.

The radiosity settings should be set as fast as possible. In most cases this is a quality vs. speed compromise. Especially recursion_limit should be kept as low as possible. Sometimes 1 is sufficient, if not 2 or 3 should often be enough.

With high quality settings, radiosity data can take quite a lot of memory. Apart from that the other scene data is also used much more intensive than in a conventional scene. Therefore insufficient memory and swapping can slow down things even more.

Finally the scene geometry and textures are important too. Objects not visible in the camera usually only increase parsing time and memory use, but in a radiosity scene, also objects behind the camera can slow down the rendering process. Normals and Radiosity   1.3.8 Making Animations

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