It's not a large difference, 1-3% delta. I'm talking ideal, not recommended or required for most people. But fact is, if GPU is splitting its attention to render desktop while also driving VR HMD, there's a small perf hit. Just like mining crypto on a GPU will be impacted by desktop activity, versus if nothing is plugged into GPU and a different GPU or IGP renders the desktop instead.Isn't the picture on screen just a mirrored image from the headset? The rendering power it takes should be almost zero.
I know because I've done a lot of A/B comparison and looked closely at FPS and frametimes. Even with a 3090 I'm OCD about having only the G2 plugged into it when using VR.