I already posted it on another thread but i will post it here too:
Low GPU usage:
Search for some low gpu usage on Google, F1 2010 has the same friggin performance bug as some are getting with NVidia cards... sadly nobody fixed it.
Another thread is that the low gpu usage was due to bad airflow on the graphics card, so it mantained a low usage to not overheat.
Into the Bulletstorm forums (UE3), one guy with ATI card was having the same issues:
It was laggy at best, only used about 65% of my gpu, and only used about 20% of each physical core, none of the virtual cores.
So, a guy told me to.
"Turn off Framerate smoothing inside the StormEngine.ini and also Turn off Mouse Smoothing in the StormInput.ini file"
Set vsync in ccc to always off and disable edge detection.
That worked, gameplay was much smoother after that, fps was now around 60, and i noticed no slowdows while i tested it during the first firefight in the game.
gpu usage had risen to 98%, but cpu usage was the same.
Stuttering:
See if you have enough Virtual Memory available. I think that may be the cause.
Take a look into your PC temps too, a friend of mine kept getting bad fps at some games because his Videocard kept getting clogged with dust.
Misc:
Disable Steam Overlay?
I will keep searching more, who knows if one of these works.