At this point I’m not sure what would be the best thing to do, first try your games and apps to see if the situation is the same on Kernel 5.4. //EDIT ok it is same.
Regarding my recent experience with drivers 390, I personally would remove them from Manjaro Settings, at this point DO NOT REBOOT, then I would follow the instructions to install the 340 drivers, and after properly installing 340 I would reboot (only at this point) and see if there is a difference now. If you have proper performance with 340, then my experience and yours could lead to more people looking into 390 to see if everything is OK in these drivers provided by Manjaro. I suspect they have issues. If it is the same then… need more 
to chime in.
If you don’t have the patience to remove/reinstall maybe wait for someone else to have better suggestion 