I checked CPU usage with Process Explorer and when D:OS is running CPU usage stays between 17-20% with very rare 30-35% spikes.
EoCApp.exe (D:OS executable) between 12-13%
(Firefox and Skype also running.)
So there's not too much CPU load.
I remember a performance issue which affected Grim Dawn, maybe it's something like
B11 Yesterday game ran great, today it's the worst I've seen.?