I'm not sure if it's a bug (on my side?) or a feature...
Everytime I alt-tab to Win 10 desktop the game keeps cpu/gpu busy (even if I enter ESC-menu before).
This amounts to ~5-10% CPU & ~10-40% GPU, +5°C & +10 watt while not playing & minimized.
On the plus side there is this nice little realtime thumb-preview in my windows taskbar

my personal
quick and dirty hotfix to stop this bad behaviour:
- get PsTools from microsoft.com
-create a batch file pausebg3.cmd:
pathto\pssuspend64.exe pathto\bg3.exe
-create a batch file resumebg3.cmd:
pathto\pssuspend64.exe pathto\bg3.exe -r
Suspend & resume are near instantaneous (windows just freezes/unfreezes the processs)
Specs:
Win 10
Ryzen 3600
Radeon 5700
bg3.exe & bg3_dx11.exe