When playing the game on my PC, it was stuttering like hell, thought it was to-do with not being optimized, since I have a dual screen, I ran task manager in the other monitor. This showed Steam.exe was using 30% of my CPU regardless.
The work-a-round I propose till its fixed will mean no Achievements whilst playing, so its up to you!
Right click Duke Nukem Forever, turn off "Enable Steam Community In Game"
Run the game
Alt-Tab out of the game
Set Steam.exe affinity to one core and set its priority to low via Task Manager.
Kill GameOverLayUI.exe if its running
Play the game and if its still stuttering set DukeNukemForever.exe to the other affinity.