Want to add the tip to [Help → Restart In Safe Mode] worked for me.
LibreOffice had been very slow to startup (Windows 10): maybe 15 to 20 seconds on an otherwise fast machine. I thought it was just because Libre is a huge application, but every other app is rocket fast, and those 15-20 seconds were getting very annoying. I considered adding Libre to my PC startup, but then that would slow down my PC startup which I don’t want to do either.
But instead when I did the tip to Restart in Safe Mode, it sped up massively starting in a couple of seconds. Then when I restarted in Normal mode, it was now fast, and all my old settings and even document history was there. Can’t understand why restarting in safe mode would make a difference when you return to Normal mode. Can only guess restarting in Safe mode cleaned up accumulated gunk clogging up Libre’s arteries?
Bottom line: Restarting in Safe Mode, just once, fixed the slow startup problem, and all my settings and document history was intact.