Upgaded project from WD19 to WD20 very slow

Hello to you all,

I converted a project from WD19 to WD20. It is and was installed in HF classic mode (3 to 4 computers) in a network. I only updated the .wdd and .exe. Now the screens with much data open much much slower.
What could be the reason?
100% the same project on the same network and workstations which was only converted from WD19 to WD20.

Anybody had the same result?
Any ideas why there is such a performance drop from WD19 to WD20?

Thanks in advance.