The refresh rate of both monitors have to be the same. I ran into the same issue years ago when I upgraded to a 144hz screen and using my old 60hz screen as a secondary was causing XIV to stutter when playing a video.

I bought a dirt cheap 1080p scepter monitor that also did 144hz and the problem went away lol.

To test if that is also your issue, change your main monitor to 60hz in the display settings within windows.