Scratch disks

Trevor.Dennis  wrote I am more and more convinced of the importance of a good Scratch space strategy to aid performance.   At the very least an SSD, and an NVMe drive if your system supports it.  They do not use defrag, which means you can share such a drive with other files.  (Unless someone knows better)

The NVMe drives (aka PCIe M.2) are so insanely fast that they remove the biggest historical bottleneck in Photoshop: writing to the scratch disk. That was always a brick wall, and you filled your machine up with as much RAM as possible to buffer, so that the brick wall didn't hit you too often. But of course there's no such thing as enough RAM, and Photoshop always uses the scratch disk, continuously.

I have one of these NVMe drives in my home machine, and initially I put only 16GB RAM in it to shave a little off the initial cost. I always intended to get more. But as it turns out I haven't bothered - I never felt any need for it. This machine handles multi-GB panos with flying colors, and it's way faster than my work machine with 32GB RAM.

That's the secondary solution (if you have more than one internal disk).

The first thing you need to do is free up space on your system drive. Most of the junk accumulates in your user account. It just piles up over time. Use something like WinDirStat to get a graphical overview of what fills up your drive, where it is and how much of it can be safely deleted (usually a lot).

When you get that message, your system drive is already dangerously full - to the point where you will soon have much bigger problems than Photoshop. Your whole system may choke. Consider it a friendly advance warning.