I agree, it is certainly slow. However, I'm not sure anything can be done about it. Can't hurt to report it, though. I didn't see any reports about this.

I just did a test render, one light, two reflective walls.

No IDL - 20 seconds.

With IDL - 530 seconds. Ouch.

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On your specific scene, does the bump map image give you really awesome realistic waves, or is it something that could just as easily be done with a node?

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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)