iPhone now hangs doing just about anything

Ever since I upgraded my iPhone 6s to OS14, the phone has slowed down to tortoise speed. Every time I want to look at emails, it has to reload the mailboxes, and won’t show any emails until it has done this. It sometimes takes many minutes just to see my email inbox, and accessing an email takes even longer. Updating my apps is now unbelievably slow. Where before, a dozen could update in a minutes or less, now there is barely 1 every 10 minutes. I could go on - every process is now a tortuously slow inefficient load. What has gone wrong, and can I uninstall the OS? Each successive OS update has worsened the problem.

My phone has 128GB capacity, and currently has about 50% used. I have cut the background app refresh to absolute minimum (not that it was ever a problem before) and it has made not one jot of difference to the cranky slow functionality