I'm not yet tanked on my morning coffee, so I have some level of built-in confusion.

You speak of a ray-tracing pass or something, that is limited to single thread. I'm not aware of such a pass.

There is the IDL pass.

There is the scatter pass.

Then there is the beauty or final render pass.

I have never noticed these avoiding 100% CPU use, but that doesn't mean it didn't happen - it means I didn't notice. What you describe is news to me. I see that you qualified it as happening only in a scene using a lot of RAM. I never do that as I am not an artist - or if I am an artist then my signature style is one light, one prop.

I will investigate what my computer does with Poser threading, but it may reveal nothing interesting.

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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)