Go to  \Documents\My Games\Fallout4\ folder  and open Fallout4Prefs with a text editor. Search:

bBorderless=1
bFull Screen=1

and change to

bBorderless=1
bFull Screen=0

It doubled my fps and removed al stuttering in a I7 4970 and GTX 660.

I LEAVE a Fallout4Prefs, butyou dont have to download it. You can download the .ini and then adjust your graphics in the launcher-settings, but dont touch borderless or full screen.

Thanks to Fluffybumbum.