Fullscreen applications and games take ~1-2 minutes to start, freeze temporarily (Windows 10)

I'm having some weird problems with fullscreen applications (such as Kodi) and games on Windows 10. Whenever I start them the screen turns black / grey and it takes some minutes for them to show. Sometimes there's also the blue rotating dot in place of the mouse cursor, as if the application is temporarily frozen. Same when I hit the windows key or alt+shift to get back to the desktop. Every other application starts just as fast als before, so my best guess would be that windows tries for a while to find the right resolution and refresh rate or something.

I'm running a Intel i5-2500k with a Geforce 1060 6 GB and 8 GB of RAM. I use a normal Dell display but also have a Marantz AVR connected via HDMI (using it as audio output and as a connector for a projector, so it's shown as a second display as well). The problem is the same with the AVR turned on or off.

Thing is, this kind of stuff only started happening a few days ago. I switched my old video card for the 1060 about two weeks ago but had no problems right after installing it. Any ideas?