Dell Precision nvidia-455xx-prime - external monitor unusable unless laptop display enabled

This is fresh install from manjaro-architect-20.0.3-200607-linux56.iso, opting for KDE and proprietary drivers.

Mostly works fine. A typical use case is to use an external monitor, and no laptop screen - and in this mode it is unusable.

If you open the lid, and use both displays, it’s fine.

If you change the display settings to disable the laptop screen, its bad again.

laptop screen alone is fine, both together is fine, external monitor by itself is unusable (looks ok, just slooooow).

I will add that cold install from ubuntu-20.04.1 yields a working configuration, so the hardware is viable.

Thanks, Clay

$ inxi -G
Graphics:  Device-1: Intel UHD Graphics 630 driver: i915 v: kernel 
           Device-2: NVIDIA TU104GLM [Quadro RTX 4000 Mobile / Max-Q] driver: nvidia v: 455.38 
           Device-3: Sunplus Innovation Integrated_Webcam_HD type: USB driver: uvcvideo 
           Display: x11 server: X.Org 1.20.9 driver: modesetting,nvidia resolution: 1: 3840x2160 2: 3840x2160~60Hz 
           OpenGL: renderer: Mesa Intel UHD Graphics 630 (CFL GT2) v: 4.6 Mesa 20.2.1 
{gni}marfa:/d/gni/ace2$ mhwd -li
> Installed PCI configs:
                  NAME               VERSION          FREEDRIVER           TYPE
video-hybrid-intel-nvidia-455xx-prime            2020.10.04               false            PCI
     video-modesetting            2020.01.13                true            PCI

Warning: No installed USB configs!