They should really just double everyone's visual carrying capacity and make it so when that bar fills you become encumbered...
Weight in BG3 affects how far someone can run in combat

. So don't load up Astarion if you want him to run around the whole map

.
what.
...does this only happen if they're carrying over their capacity? Does additional weight past this limit slow them down more and more or is it a flat decrease?
I'm not sure exactly how it works, I just started thinking about it as a problem when I noticed my Shadowheart could barely move in combat in one of my plays.
So I just spread the load up evenly now, and sell when I get back to a trader

.