I have this issue as well. I’m running a gigabyte 980 Ti xtreme gaming card with the dell 27 inch gsync panel. I first noticed it using the Discord chat app.
Since then I’ve noticed it also happens with the NZXT CAM 3.0 app as well (didn’t used to happen in 2.0 version). Also it happens in random other apps in certain dialog boxes and menus.
I’m thinking it has to be something with the way the window is drawn in Windows 10 or something along those lines. It’s like the application is unable to use hardware acceleration correctly.
I have noticed that when you turn off gsync and the terrible lag is gone, you can drag he window and it doesn’t move as smoothly as other Windows. I run a 144hz monitor so I usually notice smooth window dragging on the desktop. With gsync off the affected applications look like they are running at 30fps when you drag them. The mouse alone does not seem affected.
So I think it’s a deadly triad of 3rd party programming not playing well with Windows 10 and Windows 10 not playing well with gsync.
I don’t think this is going to be a big enough issue for ms to dive in and help but let’s cross our fingers and hope for an update that may address it.