I did some more experimenting. It dawned on me that the reflection aspect of frosted glass isn't necessary. Leaving it out allows the renderer to avoid doing an extraordinary number of rays.

I was able to adjust my parameters and get this result, rendered in about 13 minutes, which I think is really quite good.

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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)