I'm exporting a set of proofs - 1200 images at a small resolution of 2000px each (about 1/4 the size of the originals) - and my fan is running at full speed and Activity Monitor shows LR is using roughly 500-800% CPU
This is a 2012 iMac i7 Quad Core 3.4 GHz 32 GB of RAM w/3TB internal Fusion OSX 10.11.6
I've quit everything else but the essentials.
Why is LR such a CPU hog? Any way to dial it back? I don't mind the processing taking longer. I can only envision flames coming out of it when I have to actually export the full size files!
Your iMac is 3 > 4 years old and probably loaded with Dust. Try cleaning it out and do some test.
It is using a lot of resources because you (the human user of Lightroom) have asked Lightroom to perform a task that uses a lot of resources.
This causes heat, and your fans will run, and it is very possible that your old computer has several fans in the case and one or more of them have malfunctioned and you need to check to see if all of the fans are still working.