Several users have reported performance loss and/or loss of fan curve settings after installing the AMD Radeon Software Adrenalin 2020 drivers for AMD GPUs such as the Vega 64 and the RX 5700 XT. While the issue does not seem to be widespread at the moment, completely removing all traces of the previous driver using DDU and re-installing the new version looks to be a good workaround.
https://www.notebookcheck.net/Experiencing-performance-loss-after-installing-the-Radeon-Software-Adrenalin-2020-driver-Here-s-a-possible-workaround.447101.0.html