About ten days ago Big Sur was downloaded (auto update). My 2019 3GHz iMac with increased RAM 32 GB is SLOWER THAN EVER! Help! I want the rubbish removed and return to Catalina.

Try booting jnto Safe mode and then rebooting normally to see if that helps How to use safe mode on your Mac - Apple Support

If you insist on reverting to Catalina you can use your backup that predates the upgrade.

(Shame on you, Apple! An "update" fraught with problems)

Apple does not read posts here. We are your fellow end users, not Apple employees or agents.

This gets to an Apple employee:

Feedback - macOS - Apple

Before you blame Big Sur you should check to see if you have any software installed that might not be fully compatible with Big Sur and is causing the performance degradation. I'm finding that Big Sur is the best and fastest macOS that I've ever run on my 2017 iMac18k,3.

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to include the report in your reply.

IMPORTANT: ⬇︎ ⬇︎ ⬇︎

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Then we can examine the report and see if we can determine what might be causing the problem.