I too experience loss of performance when popping out 2 GTC’s of the Longitude onto my 10" touchscreen monitor, using POPM. I currently can’t seem to use VSync, which causes stuttering even if the fps drop is minimal.
Also I find it a bit weird, since the 10" monitor is running of my integrated GPU on my 12700k which i read should help preventing performance loss. MSFS is running on my 2080S. If I don’t pop out the screens, it is all very smooth with Vsync, but popping out with Vsync enabled, causes the fps to decrease by 50%. I’ve read this was fixed during an earlier SU beta, but it seems to be broken again.
Edit: I see OP is planning to use 2nd monitor for other apps only. If i only put Vpilot on the second screen, no performance losses. Only with popped out MSFS instruments/panels.