Thanks for the help.
I moved the Preference folder and it did fix the problem, but when I changed the interface from Wide to Narrow, calibre became slow again. So I did a little test and moved the old Preference folder back and changed the interface to Wide, and the sluggishness disappeared. At the moment, with my old Preference folder and the interface set to Wide, calibre works fine. I prefer the Narrow setting, but I can live with it being set to Wide.