WOOHOO!!!
This thread explains the problem and here’s the solution
From 570 changelog
Fixed a bug that could cause external displays to become frozen until the next modeset when using PRIME Display Offloading with the NVIDIA dGPU acting as the display offload sink.
but the issue is not fixed at all.
The solution is to create file /etc/X11/xorg.conf.d/10-nvidia-drm-outputclass.conf
with this content
Section "OutputClass"
Identifier "intel"
MatchDriver "i915"
Driver "modesetting"
EndSection
Section "OutputClass"
Identifier "nvidia"
MatchDriver "nvidia-drm"
Driver "nvidia"
Option "AllowEmptyInitialConfiguration"
Option "PrimaryGPU" "yes"
ModulePath "/usr/lib/nvidia/xorg"
ModulePath "/usr/lib/xorg/modules"
EndSection
Now all rendering is performed by NVidia’s dGPU and there is no freezes in glxgears/vkcube, cause reverse PRIME is not needed anymore. Ordinary PRIME (forward PRIME) is used in this setup. Yes, dGPU draws little more power, then Intel’s iGPU. But i’m ready to pay this price to get renderint without freezes. Especially while using AC power source.