Xorg actually doesn't have support for synchronizing multiple screens
very well. The compositor can only paint all screens at once, and then
the Xorg drivers have to output their areas at separate times later.
The 'modeset' Xorg driver will just sync to one and tear on the others.
And I am not sure how radeon/amdgpu are expected to interplay. If they
support a TearFree feature then that means at least one of them has to
delay each frame, which you might see as lag. Also, if the monitors have
different refresh rates then there's no guarantee Xorg will sync to the
fastest one -- it might sync to the slowest, which has to be worked
around in the drivers.
I suggest the first thing to investigate here is to see if you can get
both GPUs using the same driver (from the kernel onward). Because using
both 'radeon' and 'amdgpu' at the same time might be causing this.
GNOME Wayland sessions don't have this problem at all because there is
no Xorg and it schedules each display separately.
** Package changed: xorg-server (Ubuntu) => xserver-xorg-video-amdgpu
(Ubuntu)
** Also affects: xserver-xorg-video-ati (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1923153
Title:
Display output laggy from iGPU when operating on desktop with
attaching AMD R7 430 graphic (dGPU)
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