Are you using the right Nvidia DX version? Using DX 11 if your system prefers DX 12 will tank the fps.
no, I'm using DX12 on AMD Radeon 5700 XT and I haven't some problems caused by NVidia, fps downgrading exactly in k-version.
But have you chosen the option in the settings menu to use DX 12? If it's set to DX 11, which it is by default for some people, that would explain the framerate issues.
