To better understand what's going on I wrote the below. My experience has been RAM tends to fix slowdown with BG3.
So just for comparison I host Multiplayer games a lot and I run 32 GB of RAM. I almost never have crashes or slowdowns even after 6-8 hours of playing.
It may help to know I built my system around running this game in particular, and knowing it would be in EA.
To start with we need to talk about something called "Memory leaks" - a Memory leak is when a program utilizes a piece of your RAM to do something - create a graphic, animation, or calculate something - and then doesn't put that memory back when its done. So that bit of memory is not being used, but it has not been released back into the system, so the system thinks it is unavailable.
In a perfectly optimized game the system would always return all memory once it is finished with it, or store any relevant data on the hard drive. In EA games - with developers on deadlines memory leaks and the process of fixing them gets kicked down the road.
As your system starts to run low on memory your OS then will take a chunk of the Hard Drive and use that as a kind of jury-rigged form of RAM called a "swap file". Swap-files are pure poop. This is where your slowdown will happen since the "pipe" to the swap file is a lot slower and smaller than the pipe that goes to your RAM. So it slows everything down and increases the likelihood of a crash occurring due to the actual RAM and the SWAP file being out of sync with each other.
And that's why you need all the RAM for this game.
