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?