KB3156421 got installed on May 13 whereas my problems started around start of May, so I'd not suspect it to be the source of slow down. In fact, after I get to the log on screen, the PC seems to behave normally. But trying to follow one earlier suggestion, I ran into fresh problems.
I tried to create a local account yesterday (May 14) with Administrator rights through settings > accounts and entered a new name and password. The computer went into a huddle with the circling marbles and nothing else happened even after 45 minutes. The screen would not close and I had to do it through Task Manager. I shut down the computer in frustration!
When I switched it on today, after the same long delay, I got the log on screen and a surprise: the bottom left corner showed the new local account name (that I tried to create yesterday) as Administrator. But my original Microsoft Account name was gone! I could then use the new intended password and get my desktop screen.
So I tried to repeat yesterday's steps, this time to create a Microsoft account as Administrator but I again got stuck with the output screen asking me to wait. It looks that the Administrator is finding it difficult to create a new account! Is this in keeping with the earlier reported system error message "The Microsoft Passport Container service terminated with the following error: General access denied error"? Btw, the internet connection was on throughout and I could access my mail, update programs, run anti-malware etc.