Optimizing Phase 3 Time?

I’m doing some plotting on HDDs instead of SSD or NVME. The phase 3 adn 4 (but especially 3) is a real killer as far as time.

Could setting a 2nd temp directory not on the disk (say an SSD or NVME) help out a bit with regards to time on phase 3?

It could, but only if your ssd is bottleneck, as in writing more than it can handle.
I am running all with second temp on hdd and see no significant difference in plot time.

But that just means that my cpu is the bottleneck for me and not the ssd.

It does mean the ssd is writing less, so will last longer.