Fallout 4 Lagging but not using full PC potential

Hello. I've been playing a lot of Fallout 4 recently, and I find that whatever setting I play at, even at Mid-High graphics, I usually get 60fps but in the really populated areas I get down to 45 or 37 sometimes. When the FPS goes down, the max GPU usage is about 45-54% load and the CPU is only about 80%. Is this a game or my PC problem?

- Sam

I've been running it on Low settings, with textures at high and godrays and shadows off.

PC specs:

i5 4690k

8gb RAM

r9 270x 2gb.