Perhaps the easiest thing to try is to add a small, cheap and fast SSD. Then you can have the calibre database(s) on the SSD but keep the books themselves on big, cheap and slow spinning iron on the HDD.
And you could also put the TEMP-folder and swap-memory on the SSD.
So you don't need to buy an expensive huge multi-TB SSD to speed up calibre. It is sufficient with a small one with room for just the database files (perhaps times two or three to allow for maintenance work, TEMP and swap).
Also you might consider updating to a free high performance modern operating system. I run the latest Ubuntu Linux. These days Microsoft supports running Ubuntu on a Windows box, but that is not very performant. Better to ditch Windows altogether.
