WHAT THE HELL!!!
I'm running a 1x1 grid (v18.65) with lots of islands, and it took 10GB RAM at 125-140% CPU with 0 players on a i7-4770K on a Wine/linux box (percentage is aggregate across cores)
I tried the NetServerMaxTickRate=15 trick, and it dropped the usage to ~70%. Nice!
Then I read your post, and went "naaaah, really?"
Changed my Day0 in ServerGrid.json, just changed the 2019 date generated by the tool to 2018. Usage is now ~25-32%...
WTF!!! You da man!