Stefan was telling me about the transparency problem. I don't fully understand the problem myself, as even after hearing about it, it still seems possible to me that it should not be that big a deal.

Firefly is actually two different renderer strategies combined - REYES and raytracing. In REYES, transparencies are not processed in and of themselves. Rather there is some sort of back-to-front rendering that doesn't involve a "global view" of the geometry. (Again, I don't fully get that.) But when processing a ray, hitting a transparent surfaces is like tripping while running. The renderer finds that surface but has to launch another ray which starts the whole "global view" things again. (I know I'm not being clear, but I'm not clear on this myself.)

If you pile up 6 layers like this as happens with some hair, you get a lot of tripping. You get a similar slowdown if you set up 6 layers of refraction and try to render through it. 

I think the case may be that to completely fix this would require that Firefly be discarded altogether.

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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)