Photoshop uses RAM to process images

Photoshop uses RAM to process images. If Photoshop has insufficient memory, it uses the
connected storage device designated in its Preferences as a “scratch” disk to process information.
Therefore, increasing the amount of RAM in your computer will enable you to work on more and
larger images inside Photoshop.
How much RAM is enough? Watch the Efficiency
indicator (available at the bottom of each image’s
window) while you work in Photoshop to determine
when Photoshop maxes out on RAM and starts using
the scratch disk, which slows performance. Click the
pop-up menu at the bottom of the image window and
choose Efficiency. (You can also view the Efficiency
status in the Info panel.) If the Efficiency value is
below 100%, Photoshop is using the scratch disk and,
therefore, is operating more slowly. If the efficiency is
less than 90% to 95%, allocate more RAM to Photoshop
in Performance Preferences or add additional RAM to your system. 4GB will cover most digital
photography uses; 8GB leaves room for other apps and fits huge documents in RAM.
If you are unable to add more RAM to your computer, then install or connect a fast drive and
designate it as your scratch disk. An SSD (Solid State Drive) is the fastest solution; an external
RAID 0 system connected by Thunderbolt, USB3, or FireWire 800 is a workable alternative if
you are unable to add internal storage.
Photoshop performance is discussed in greater detail in this Adobe Knowledge Base article
(tinyurl.com/AdobePWP-11).