I found the thread https://forums.developer.nvidia.com/t/high-cpu-usage-on-xorg-when-the-external-monitor-is-plugged-in/169173
Though the symptom on this thread was high cpu usage on hybrid graphics, the underlying symptom was the same, fast monitor requeries. Unfortunately, no solution found besides that this was popOS specific.
You could try disabling the power-daemon and plugin, which was my last guess in that thread.
Fractional scaling is Ubuntu specific, I don’t have experience how well it works meanwhile and its impact on performance/vmem usage.