extremely slow performance (i-mac pro 27 inch) after upgrading to Catalina 10.15.6

Well I see 2 major issues. First the computer only has the original 8GB of RAM, this should be upgraded using either Crucial.com or Macsales.com RAM. I would install an additional 16GB or 32 GB or RAM into the two empty slots in your computer. This is very easy to do, it requies no tools and can be done by just about anyone in less than 10 minutes.

The 2nd thing I see, is there is no evidence you backup the computer. This is a 100% guarantee you will lose data. At the very least you should have an external HD (2-3 times the capacity of the internal HD) and use Time Machine which is pre-installed on the computer. I recommend one of these EHD's https://eshop.macsales.com/shop/owc-mercury-elite-pro and you can learn about Time Machine in https://support.apple.com/en-us/HT201250

Finally, your computer is 6+ years old, this means it is getting to the age when a HD can begin showing signs of failure. I am not trying to scare you, but you should be backing up in the event that does occur otherwise all of your data (photos, music, important documents, apps etc...) will be GONE. Your HD may be good for another 6 years, but operating the machine without a backup plan is very shortsighted!

Since we know nothing about how your computer is equipped, it is hard to respond. Apparently Catalina does use more RAM than other operating systems. You may have been operating with Minimal Ram and the upgrade tipped you over the edge. Do you have any antivirus apps or Mac cleaner apps installed? If so, remove them as they are completely unnecessary. We recommend that you download and run the free app EtraCheck, allowing full disk access. Then you can post the report down below under Additional text. That will usually enable some useful observations.

No such thing as late model 2014 iMac Pro, the iMac Pro first came out in December 2017. You mean regular late model 2014 iMac 27 inch.