I did open another console.  I couldn't see anything obvious.  What are the tricks to finding a zombie process ?

I let dnf update complete and it did, successfully.   When I rebooted the main console wasn't completing to a login, so I knew something was up  !  However I was able to get a login at a second console, so I did. 

The graphical session never did boot for me, so I had no idea what login was being used, so I tried a few things.

systemctl disable plymouth.service --force
systemctl disable graphical.target --force
systemctl disable lightdm.service --force
systemctl disable kdm.service --force

Then I did a bit of investigating

ls -l /etc/systemd/system/display-manager.service

This told me it was running sddm.

so: systemctl- disable sddm.service  followed by shutdown -r now

Now it boots to a command line and runs at a reasonable speed !

I don't need a GUI right now anyway.  I'll investigate further when I get some time.