If you’re using Photoshop to work on your image then switch to another app like Safari, Mail, InDesign, etc., then switch back to Photoshop, you may notice that Photoshop gets a little laggy or even stuck.
Conventional wisdom says you need more RAM. Unfortunately that is neither cheap nor possible with most Macs. A fast SSD drive will help, but again it’s neither cheap or even possible to upgrade your storage drive on most Macs anymore.
The solution might be found in Photoshop itself.
Go in to your PS Preferences (Command+K) and choose the Performance tab from the list on the left. Once in the dialog, tick the Use Graphics Processor checkbox if it isn’t already checked, click the Advanced Settings button. Change the Drawing Mode drop-down menu from Advanced to Normal (if it’s already set to Normal, change it to Basic). Also make sure Use Graphics Processor to Accelerate Computation and Use OpenCL/GL are checked. Hit OK and you’re done
This will tell Photoshop to use your Mac’s video card to help with the heavy lifting, but not to over-do it.
While you’re in the Performance tab, you might also want to set the Memory use to about 70%. Over the years I’ve found that using much more than that of your total RAM for Photoshop has more negative effects than positive ones.