MacOS Ventura frequent "spinning beach balls"

Lately, my iMac (Retina 5K, 27-inch, 2017) has been very unstable. Even the simplest action throws up a spinning beach ball - just in the last half hour, I opened a HEIC file from my phone with Preview, did some simple edits, and tried to export to a jpg. Spinning beach ball. After several minutes, I did a force quit, and tried again, same results. This has been happening much more often these days, on programs big and small (Preview, Finder, Lightroom Classic). I have almost 1/2 of a 4TB disk available. I have 64GB of memory. I have restarted, shut down and restarted... Generally nothing looks odd in the Activity Monitor. Any thoughts? This is driving me batty.

iMac Retina 5K, 27-inch, 2017

Memory: 64GB 2400 MHz DDR

Startup Disk: Mac SSD - 4TB, about 2TB free