Quote - That sounds like you're disk-thrashing, if Firefly is close to its 3GB limit. I don't bother letting renders finish when there is swapping happening, because the time you're spending now is not for quality.

I've upped the memory in my old white iMac to the maximum, which is 3GB usable. Then I set an IDL render running having given this thread on RDNA a good read (along with some of the comments in the previous "IDL for Dummies" thread here). I monitored memory and CPU use. The CPU was pretty much maxed out most of the time, and there was very little page-swapping going on (about 400 page-ins and no page-outs in 8 hours). I conclude that IDL is just very computationally intensive, so if you're going to use it, make sure you've got a machine with plenty of nice fast CPU cores (and enough memory).