It’s not just graphics, if anything BG3 is light on graphics horsepower needed, and from a quick scan around is a bit heavy on RAM.

From previous developer comments part of the issue seems to be the world simulation plus split screen. If you split your party the whole current map has to be running/simulated all the time, you can’t freeze/unload different areas as you walk away, and that includes keeping track of objects for physics - ever notice how bethesda games don’t have many physics objects outside? Think along similar lines to a dedicated server that runs without graphics - just all the world, characters and object meshes, or a large world MMO that has to keep track of what players are doing at different ends of a continent.

Adding graphics back into it, when the party splits up they’re probably looking at different things increasing the breadth of content that has to be loaded. They might be able to cut down textures, but it could mean being more brutal with cuts than Larian wants because they don’t want their game to look like shit.