Peripheral vision helps with anchoring where and what something is. If what I'm looking at suddenly changes to something else, there's a moment of disorientation, too brief to react to, but still enough to interrupt my train of thought. If pressing cmd+tab didn't actually produce the very next thing I needed to see, this is even worse, since now I need to consciously decide if what I'm looking at is the right thing, press cmd+tab again, wait until I understand what I'm looking at, etc. I feel confident that this is not a problem unique to me, due to the existence of extensions like https://marketplace.visualstudio.com/items?itemName=johnpapa... , which colors window borders differently to allow reducing this delay.

If I turn my head to look at a window that I could vaguely see in the edge of my vision, there's no such disruption, at least consciously. It feels as though turning my head doesn't require any conscious attention at all, unlike the "what am I looking at? Is this what I expected? What was I thinking?" process that happens with hidden-to-visible transitions. By the time my head has turned to bring the window on the other monitor into the center of my vision, I already have the visual context, and in the meantime, I haven't lost my train of thought.

This. Plus, can it really be referred to as a "study" with just three test subjects?

It doesn't feel as seamless as just glancing to the side. Plus there's some annoying little context switch feeling to it