from October 2018:
"If you already have an SSD as your main drive and have your games and large files on your mechanical drive, it is still worth getting a second SSD to use as a cache for the mechanical drive. This small investment to get either a 32GB or a 64GB SSD and use it as your hard drive’s cache is going to be worth your money for the performance gains.

In fact, since SSDs are becoming cheaper and cheaper, there should be no reason for you not to get a 64GB SSD to use as a cache memory for your HDD."