So ever since creators update, I've been having this problem and so far I havent found a way to fix it. It's almost like windows does not switch all GUI task over to the dedicated GPU in "Show Only Monitor X" mode. Only happens on external monitor, as soon as I launch a fullscreen game, it will stutter a few seconds then go to full FPS. Uploaded from phone, sorry for vertical bars.
Laptop Spec:
MSI GE63 RGB 8RF
i7-8750H - 6 Core
Intel 630 + GTX 1070 @120Hz
16 GB
1TB M.2 SSD + 1TB HDD