Mac Mojave Sys Prefs/Screensaver slows down, locks up
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.
I'm having the same problem here, and I can confirm that it definitely began after the most recent Security Update. I will put in a bug report as well.