Multi monitor setups often don't work 100% issue free especially for gaming on one monitor and playing videos on another. This gets even worse when monitors are not of same refresh rate, etc.
This all has to do with GPU drivers and how the OS handles rendering on multi monitor setups. It has never worked right. Sure it's fine for looking at windows of text but it never works well for playing games and videos where you will see these hitches and stutters.
Some apps will even outright crash if you drag them between monitors of different refresh rate, yet work fine if refresh is equal, and so on. The oddities and issues are endless.
Want to play games and videos... get a single monitor setup.