Changing focus sometimes causes the game to appear to freeze, but I've never had it crash because of this during gameplay. The game consistently pauses when focus is lost, so if you change focus and find the window is no longer rendering, hitting escape repeatedly will likely wake it back up. There is an exception: if you change focus during startup (i.e. during the content warning screen), the game will actually crash with a DirectX error.

I occasionally encountered an issue where the mouse would get laggy and cause the camera to move with a delay, but this would go away just by aiming for a bit.

The most significant issue I encountered was hitching when entering new areas or at the start of some setpieces or cutscenes. This becomes less frequent as the game goes on, and the game handles stuttering in general pretty gracefully, so this never got in the way of gameplay.