After months of investigation without any success, and any useful guidance from C1 Support I have to say, I decided to perform a costly clean re-install of Catalina on my Mac Pro 2019...
And I have good news... after a careful planning and a few hours of work I was able to re-test the export performance of C1 Pro 20 on GFX-100 files and... it improved from 25s per file to around 1s...
I guess that I will never know what the problem was.
Happy for now, but I am still amazed at the incompetence of C1 support who was completely unable to help me diagnose this issue.
As far as import goes, it still isn't as fast as it used to be, but I was able to significantly accelerate performance by creating a new Catalog. It's now manageable.
The real question is, will I ever be able to gain back in runtime performance the time I have lost? Most probably not.
Cheers,
Bernard