Mojave screen saver

Interesting problem with the Screen Saver function in Mojave.  Several months ago I noticed two related problems.  I’m using the Classic option pointing to a large file of about 350MB.  When the display moved to begin the screen saver, the display went to a gray screen for 30 seconds to a minute, then started the screen saver photos.  During this time, the computer was unresponsive to mouse or keyboard.  When opening Screen Saver in System Preferences, no file displayed for also 30 seconds to a minute, with “loading” displayed.  When the loading finished, System Preferences was usually frozen and had to be Force Quit.

After struggling with this for some weeks, I reinstalled Mojave using Recovery.  That fixed the problem but some days thereafter it came back as before.  At this point I began to wonder if it might be associated with a Security Update.

Yesterday I again reinstalled Mojave and without thinking immediately installed Security Updates -005 and -006.  (Apparently Mojave restored without including these updates.)  This time the problem was still as before.  This morning I again reinstalled Mojave but did not add the two updates.  The problem has gone away.

The problem-free Mojave is 10.14.6.  Can’t find the Build #.  The iMac is 21.5”, Late 2013.

It seems pretty clear that something in the updates is interfering with Screen Saver, probably with -005.