IDL with transmapped hair is a lot of calculation.

When a ray is sent to sample the luminance on a particular spot, hundreds of other rays are sent out from there to find what other objects are lighting that spot.

Those in turn send out hundreds of more samples, for each of those original hundreds.

Now when a semi-transparent polygon is sampled, it not only does that for that point, but it shoots an extra ray through the point to see what should be added for what is seen behind that spot. This in turn starts the process all over again. Worse, the second set will bounce back and hit the first set, but from behind resulting in calculations for back-facing polygons. The inintial release had some issues with IDL on back-facing polygons, so the problem is exacerbated.

The amount of work necessary to calculate IDL for a small patch of hair can equal or exceed the work necessary for the rest of the room!

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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)