Reboot the computer and make sure that PD runs GPU optimization when it's started. You can check to see if it already ran and/or manually run it from the Preferences | Hardware Acceleration menu.

Worst case, uninstall the nVidia driver and perform a clean install as described here. Even if your son is a gamer, start with just the video driver to see if the problem goes away, then you or he can add any additional drivers and apps (like GeForce Experience) one at a time and make sure they don't slow PD down.