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.
Do you have Steam pre-cache shaders before the game opens? You can turn that off in the settings and have it compile them in the background while you play. Depending on the game, this can dramatically speed up starting time.
However, given the nature of Factorio’s graphics, I’m not sure how much of a difference it’d make.