Same problem
Low GPU utilization

System
Intel i5 3570
Palit GTX 980 4gb (ref)
8Gb RAM

In game settings - Ultra
50% GPU load in the menu and avg 50fps
20-30% GPU load in the any singleplayer mission and 20-30fps

MSI Afterburner monitor screenshot added
First half - in the menu
Second half - loading and playing singleplayer mission