big sur – Big Sur update causes beachball icon frequently and Finder to be unresponsive

I’m on a MacBook Pro , bought June 2020.

I updated to Big Sur from Catalina 10.15.7 this morning, and the upgrade went OK.

However, since then, it’s become slow and unresponsive, beachballing a lot, Finder had problems launching and constantly Application Not Responding.

Finder won’t launch very often, if at all.

It’s the latest version of Big Sur, 11.3.1, which I’ve got.

Other problems are that I cannot click on the black Apple Icon to shutdown or restart (sometimes it works, sometimes it doesn’t), and apps crash a lot – BBEdit, occasionally Firefox etc. have Application Not Responding.

I tried creating a second user account with admin privileges, same issue there; repaired user permissions for my main admin account.

I did an EtreCheck and had to remove remnants of DiskDrill which I’d tried when I had Catalina.

What solutions would you recommend, I’ve tried Google-ing but aren’t sure what would fix this?

iTunes Spinning Beachball after the update of Airport Express; Mojave iMac

This may have nothing to do with the Airport Express update, but … When I open iTunes and click on a title, this beachball is displayed. Sometimes I wait and finally force the quit and start over. The beach ball then appears, but finally goes away. The beachball sometimes appears when I click on another song. This happens regardless of whether I'm trying to connect to my Airport Express. Sometimes I don't even turn on AE. I restarted both the iMac and the AE. I'm now wondering if I should delete iTunes settings (if so, which ones?)

This problem does not occur with my older MBP El Capitan

iMac (Retina 4K, 21.5-inch, 2019) Mojave

Airport Express version 7.8.1

iTunes version