Comment 15 for bug 1940247

NV update for NV#3374575:
Hi Kevin - This is an expected issue for all drivers with the currently shipping X server (1.20.9 in the bug report): https://gitlab.freedesktop.org/xorg/xserver/-/issues/1028 . https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1875015 .

The issue is that X Present doesn't support synchronizing to PRIME heads. When the external display (via Reverse PRIME) is the only head left, X Present vsync apps have nothing to sync to and throttle to 1 FPS by default. The only solution right now is to disable vsync, try adding a .sh script in /etc/profile.d with the following contents:
export CLUTTER_VBLANK=none
export vblank_mode=0

That should fix the 1 FPS issue but obviously you won't have vsync. If the system is muxed such that external displays are connected to the iGPU in this configuration, you won't have the problem.

Thanks and hope this helps, THX