Hi Carlo : I have done a cleanboot as well as the Repair Upgrade as advised by you but to no avail. The system is just as slow as before. Programs take ages to launch and browsers (be it Edge, Firefox, Opera, Brave or Vivaldi) not only take a long time for pages to open but most of the time it says 'Not Responding'.
This reply took over 45 minutes on the Vivaldi browser before submission of the post succeeded . Clearly something is not quite right with the operating system.
It appears unlikely the problem could be due a virus as I have run Malwarebytes, Windows Defender and SuperAntiSpyware and no threats were detected. My computer is absolutely clean.
In the circumstances, I am considering going back to the previous version of the operating system but is there anything else I can try first?