Just to note that I found the root of the problem - inadvertently, on a different machine. It is LibreOffice's QuickStart utility. The machine was shutting down very quickly, until I turned LO QS on. Then, molasses. It took me a few days to remember what I had changed, and then as soon as I disabled it the problem was gone.
BTW, QS is deprecated in LO, and in the not distant future will be eliminated.
Thanks again everyone for the help.