One more time, I want to point out that the raytrace bounces you set does not guarantee that it will be used. If your scene has only a few opportunities to do two bounces, then it's really not going to matter if you allow 20.

I changed my props so that they have no opportunity to allow a ray to enter either prop, by removing the transparency and the refraction. So the only bouncing is reflection.

There are no other reflective objects in the scene. Almost all bounce paths are single bounce. A small fraction are double bounce where one prop reflects the other prop into the camera. That's two bounces.

This same scene, still rendered with 20 bounces in render settings, took only

21 seconds

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)