Yes, I forced the upgrade, using the media tool. 21H1 was not shown yet in the Check for updates area.
Anyway, it was really something very strange, because after updating the BIOS things seemed better. After I tried to restart for a couple of times, the same slowness returned.
I ran FRST again and noticed that in the Errors section of the log two errors were repeating themselves: The first one was about the Bonjour service and the other one about a XTU3SERVICE. I uninstalled Bonjour and searched about the other service, to find out that it belongs to Intel services and has to do with a kind of overclocking.
I did what this link says about this:
No other sign of it elsewhere.
The next start-up times were/are perfect, around 1-2 minutes. However I didn't know what helped. I ran once more FRST and again in the Errors I saw the XTU3SERVICE error (failed to start), so I gather that uninstalling Bonjour and updating BIOS did the job.
I will search a bit more about that service and see if there is anything to be done to get rid of the error. Any ideas or do you think I should leave it as it is since the problem is solved?