Windows 10:
Crucible: Memory would climb to around 8.5GB and stay there for a while before dropping down a bit.
Breach: Memory climbs gradually until over 10GB and then starts dropping down until its below 5GB
If windows is moving too much of the game’s ram into the swap file, that could drop the games performance.
Linux:
Crucible: Ram stays below 9GB
Breach: Ram keeps going up and then performance drops.
How to reproduce:
Keep playing the game while watching the ram usage.
Teleporting between whispers and Sacrament can encourage this to happen faster but still do some moving around from area to area.