I am having the same problems. Just loaded up Archicad 20 today for the first time and 3d is barely usable. Loading time did not seem to be an issue but I have the same exact file opened in Archicad 19 and Archicad 20 and the difference is significant when moving around in 3d space.
Attached is the GPU usage charts from both programs just rotating around in 3d its seems that Archicad 20 is not fully utilizing my GPU. Not sure if this is the issue or if this is a result of something else that is causing the problem.
I will run additional tests on the other office machines when time allows. As it stands now, I cannot migrate my projects to AC 20 with this drop in performance.