Something that I have noticed is when you power on a settlement the fps takes a massive hit and stuttering is very much noticed. I took a mission to kill scavs and while killing the scavs my fps was close to 60fps average, after killing them I used a Power reg I had to power on the system so I could loot the settlement. Soon as I used the power on terminal the fps tanks to the 20's and stuttering. I collected all the loot and then powered down the settlement to get my regulator back but the stuttering and low fps was still there. Making driving the SRV back to my ship almost impossible.
This needs looking into because the only reason to go to settlements is for the Odyssey DLC, if we can't do that then it renders a large part of Odyssey unplayable.
*Additional info edit
I just did a mission to restore power and the settlement was not on fire the fps in this situation was not as bad as was playable, so it would seem that the fires/reflections/water seem to have a major performance impact. The combination of all three having the most impact, it seems when you power on the settlement the reflections of the fire and the lights coming on is just killing the performance.