Have you checked your gpu temperatures? This can happen if your graphics card isnt being used properly but there are quite a few things you can try. First, make sure youre not hitting any frame limits since theres actually a hidden 120 fps cap that stays on even if you turn it off in the settings. If your cpu usage is super high while gpu stays low, you might want to enable xmp in your bios for better ram speeds, and try turning down population density a bit.

Something that worked for me was switching to directx 10.1 and keeping grass quality on normal, you'd be surprised how much of a difference that makes. Also worth dropping anti aliasing down or turning it off completely while youre at it. A clean driver install using ddu might help too, and dont forget to set your power settings to maximum performance in the control panel.

Good to know: if youre running a really beefy card like an rtx 4080, you might still see lower usage than youd expect, its just how the engine works sometimes. Make sure vsync is off and youre running at your monitors native resolution. Closing background apps and checking for thermal throttling with msi afterburner can help too, especially if youre seeing your cpu maxed out at 90 100% while your gpu is barely breaking a sweat.