While my Operating System is installed on a SSD, i install games like Factorio on my HDD. When running on a hard disk, Factorio takes a long time to startup and show the main menu. A significant percentage of that time is spent on IO, reading the game assets from the slow hard disk.
You shut Factorio down?
Seriously though, Factorio isn’t too large, why not keep only that game on the SSD?
My SSD is mostly full. Factorio is only slow at startup while its loading the game, after that it wouldn’t matter whether its on SSD or a HDD.