Had the exact same stuttering on Arch Linux with BG3 - only when moving the mouse/camera after an hour or so of gameplay. Resolved it completely when I disabled the new Steam background recording feature and Steam overlay (on BG3 only). Not sure if it was just one of those things or both, but it hasn’t happened since turning them off. Might be worth a try for you too~

Edit: Oh, also I have these launch options, I think the LD_PRELOAD="" might have been related to this issue too~

LD_PRELOAD="" VK_LOADER_LAYERS_ENABLE=VK_LAYER_MANGOHUD_overlay_x86_64 %command% --skip-launcher --vulkan

I love how happy they both look 😸