From Adobe:

If you have more than one hard disk, you'll get the best performance when
virtual memory is on one hard disk and the Photoshop scratch disk on
another, so they do not try to use the hard disk at the same time, which can
slow down Photoshop and the operating system. However, if the operating
system has enough RAM to not have to swap pages out of RAM, then having
virtual memory and the Photoshop scratch disk on the same drive will not
decrease performance.