Mac Mojave Sys Prefs/Screensaver slows down, locks up iMac (late 2015), 10.14.6 -- opening Sys Prefs/Desktop &amp; Screensaver, all actions slow down. Switching between Desktop/Screensaver, changing Screensaver options, etc, takes a couple minutes, occasional spinning beachball, locks up all other apps until Sys Prefs finishes whatever it's doing.  Moving the cursor to a hot corner to engage screen saver similarly requires a long wait while it loads the photos for the screensaver. I've changed the source of the screensaver images between my own photos and system images, no difference.
The problem appears to be a long delay in screensaver loading images. I assume something is corrupted? Anything I can delete/replace to fix this annoyance?
TIA for any help.
Problem was introduced in a previous update and seemed to affect Macs with multiple screens.
Latest security update (mid Dec '20) seems to have fixed it. Has for me anyway. Screensaver loads in just a few seconds and System Prefs. loads the preview and allows edits without crashing.