One can speed up Photoshop by moving the scratch disk to some other partition which has more free space. It can be some other drive, other than your boot drive. It’s recommended to set Scratch disk in a drive which is not your boot drive (the drive in which operating system is installed).
In simple terms, scratch disk is like a virtual memory and this is used when physical memory (RAM) is full. For best results always set your Photoshop scratch disk which are fast drives, whether internal or external.
To move/change scratch disk open Photoshop and go to Edit > Preferences > Performance
In this screenshot, D: is selected, as C: is the system drive and also free space is very less in it. Therefore D: is selected which has more free space. You can also select more than one drive and arrange it in priority as per your wish. Just select it and click on that small arrow button. Finally click on OK and then settings will be applied upon restart.
Another small tip, notice the Memory Usage section. You can even increase that to improve the performance. E.g. in this screenshot it is 977 MB. You can change that to 1200 MB also.
So that’s it. Go ahead and improve the performance of Photoshop.