I just retested on 3 machines and none of them have any issue with rigging, and all use multiple core processing while at sea. I tested a battle with 20 ships in the ERAS gold fleet and everything was fine.
Machines tested:
Win7 laptop, Intel i5 dual core, embedded Intel GPU
Win10 laptop, AMD Ryzen 5, Radion R4
Win10 desktop, AMD Ryzen 3, GTX 1060
I can't fix what I can't reproduce on my own machines and none of them have this problem you show and all of them utilized multiple cores to distribute processing.