Isn't it the split screen causing the issue, I don't think the PC version has that. To be fair, the game does play on steam deck, but you have to knock some graphics down and lock it to 30 for a stable experience. Personally I don't think all the panic about the series s is justified, some Devs like CDPR have done amazing optimisation for it and like you say, games will be designed for less powerful PCs for a while yet based on stream surveys.

Obviously there's a bottleneck in BG3s design and the S hardware somewhere they're trying to solve with MS to get the game out. It's unfortunate that it means MS miss out on a surprise hit of the year at launch, but it's not like their players are short of an RPG to play very soon.