Reading all that I can only confirm and beg Larian to act on what is happening to split screen coop performance.

I'm playing with my wife and we're like ~45 hours in. It's haaaard. So many actions cause lags (it was all mentioned here) and we endure because we love the game and story so much, but definitely, the experience is not great because of it. If it would be any other game I think we would drop it and wait for a fix instead of pushing through it.

Lack of a dynamically merging camera is also a huge miss, especially after playing Divinity Original Sin 2, which had a great coop split screen experience.

Larian if you're reading this, please address performance issues ASAP. Having a merged camera would be such a great addition to couch coop too.