Don't take this as a guide or anything, this was just the result of an evening googling and trying stuff, the game still runs like crud in comparison to Witcher3, but it's a lot better than it was. My 6700k rig suffers the same kinda thing, if anything compounded by running SLI.
Fo4 isn't very well coded, it puts far too much load on the first thread so any time you're in a city or very built up area the FPS will plummet, GPU Utilization will be less than 50%, I saw 30% a few times while thread 1 was ~70-100%, the rest of the threads weren't doing a lot. So I used the 'Fallout 4 Configuration Tool - By Bilago' to change the # of threads from 4 to 16, and then you need to open the console and type tMta ON, there were a couple of others recommended but they seem to be permanently stuck on OFF.
Link to "How to enable Multithreading options" on steam forum.
After that it seemed to be spreading the load out more, all threads are actively doing stuff now. FPS still drops in certain areas but not to ~30fps but more like >55fps min. When gpu is at 99% util, it's usually 100-120fps, 1440p all settings maxed. (actually it's worth pointing out that this is with a few mods to make the wasteland look overgrown and vibrant, so my fps will be a lot worse than with unmodified game)
it runs beautifully in most settlements and the wasteland, but just watch thread 1 and gpu utilization when you go into a densely packed city or something. I think you can change the thread count in one of the .ini files, but it's easier just to use Bilago's Configuration tool. The annoying bit is you have to enter the tMta ON command every time you open the game, I don't think it stays set. (should say "Multi-Threaded Accumulation is ON", if it says off just repeat the command. For me to get in console is the @ on the uk keyboard, but ` or ~ might work.
