Numonex:
I have been doing a lot of research. You can evidently downgrade. It requires making a complete backup, erasing the mac, then restoring the computer to its status right before Catalina was installed. Deciding between that and throwing the iMac off the balcony was tough, but I found what might work for me on an interim basis: I have the iMac AND a MS Surface laptop, on which the MS Office apps are still lightning fast. I set up file sharing on the iMac just now and am going to wait and see if Apple fixes the problems they've created. The main things I lost or could not use with Catalina were Adobe Acrobat Pro X (DOA after Catalina) and MS Office (sometimes working, sometimes not, always at a snail's pace). With file sharing, I can access documents on the iMac, use and create pdf files, use the Office suite and move back and forth a lot more efficiently than waiting for "verifying files" to happen. If Apple and MS get together on this, the Office problem might get solved with an Apple patch. As for Acrobat Pro, Adobe was updating Acrobat for all prior MacOs changes, and the program seemed to work well. However, Adobe may have decided simply to ignore trying to update/upgrade for Catalina. Instead, you can only use Acrobat on the new OS if you buy Adobe's new version for $500, or subscribe to Adobe. However, Adobe has spawned a lot of competitors with cheaper alternatives to Acrobat that work on Catalina, such as PDF Studio Pro, a program I was already happy with before Office quit working right.
Sorry to be so wordy, but I've spent three days arriving at my own "wait and see." Without the MS laptop I might have opted for the balcony option. Meantime, let's see what Apple does...
Mike