This is a long time bug that got somewhat fixed in a previous release, but then reverted back. It is better (for most) on DX12.
Using nVidia Gsync amplifies the issue as it splits the frames in half for every screen you pop out so I don’t recommend using it.
With my setup (5950x/3090) I can run the CRJ with 8 pop out windows and get ~50FPS @ 4K with DX12.
DX11 it’s much worse.
Your mileage may vary.