Comment 1 for bug 1947820

https://bugs.launchpad.net/nvidia-drivers-ubuntu/+bug/1940247/comments/15

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.