Well, something is running in background. While CPU can show essentially no activity, there are dozens of background processes in a running MacOS. These typically consume 2-4 GB memory, even if no other app is running. The main thing is, when you're trying to do something in PS, does the Swap Used number increase?
You're right, something is running. But just system apps, pc is new, I haven't even installed the little count of apps, cause I wanted to use photoshop as most important app for me. You asked about SWAP number. I've just tried to do something in PS, and don't see any difference in swap size number (40mb).
iCloud, images are stored on SSD, because I wouldn't be able to edit or open them before I hadn't loaded them (or one of them handn't been loaded). Yep, I've tried to open any image from SSD folder, and I've got same bug..
Create some new files and put them locally on your drive. Check out the settings for photoshop and try making sure you have enough scratch space, access to RAM and all the typical tweaks associated with Photoshop. Once you have done this, try opening up only local files and see if there is a difference. Let us know what your results are.
Made the highest preferences, didn't see the difference in work.
Take a look, guys, what I've realised. I turned on the graphics card in settings, reloaded ps, created picture (1000x1000) with white background, and painted with the brush. I saw that brush was moving smoothly. See next, I made the same, but with another pic size(2000x2000), and I saw how the brush was really slow. See next, I turned off the graphics card in settings, AND there was no difference in the same pictures, I got the same slow brush in picture which was 2000x2000 pixels.. Looks like PS is not working with my video card(I mean radeon) at all.