I have buff enough PC to run game at ultra everything fullscreen (it drives my card above 90C so I'm usually running at middle, I can't see much visual difference beyond that anyway). Yet, the game "drops to 0fps" for about half a second every few seconds when playing in window. Not when I stand still and only moving the viewport tiny bit to test responsiveness - only when quickly rotating or moving. I mistook it for not having enough graphical power when I was first trying the game, I go for windowed by default, but process explorer show that the GPU usage drop down to 30-something % during the nonresponsive time (normally it runs at 85% GPU).

Workaround: turn off vsync when running in window