This eliminated all* stutters for me in DX12
*except in cutscenes

Do the following:
1. Go to
Windows Security > App & browser control > Exploit protection settings > Program settings > Add program to customize > Choose exact file path > add \The Witcher 3 Wild Hunt GOTY\bin\x64_dx12\witcher3.exe > Control flow guard (CFG)/Override system settings to OFF

2. Disable ALL overlays (GOG, steam, nvidia)

Note this does not fix other performance issues eg FPS
Have fun!