Now you know my problem, you need a top-end computer just to run the game smoothly when in Boston. For me no graphical quality settings helped, it's the CPU. The engine is struggling to render next-gen graphics efficiently with its ancient framework, unlike something like Red Engine 3. Even i7s and FX9590s are having FPS problems in the city apparently.
I've been testing the ini tweaks to see how I can squeeze out performance and make the game playable in the city on my 4 yr old PC. Here's some tweaks under the General section of my Fallout4.ini , so far it's stable and has improved my FPS quite a bit after a month of experimentation. Be sure to backup and test these one by one, as they may hurt your performance (but for me I've tested with/without and this is the best so far):
bUseThreadedAI=1
(unsure of the gain here)
iNumHWThreads=4
iHWThread8=3
iHWThread7=3
iHWThread6=3
iHWThread5=3
iHWThread4=2
iHWThread3=2
iHWThread2=2
iHWThread1=2
iAIThread2HWThread=3
iAIThread1HWThread=2
iRenderingThread2HWThread=1
iRenderingThread1HWThread=0
(Boris recommends against this, but for me, it definitely helps. What the above tweaks do (in Skyrim as well) is 2 cores dedicated to rendering without any other touching it, it helps alot in Boston, and it doesn't seem to cause any crashes for me - obviously you can play around with this depending on your CPU. It seems on default the cores are processing everything at once instead of being dedicated to tasks)
fProcessListsUpdateHighFrameRate=30.0000
fProcessListsUpdateLowFrameRate=22.5000
fProcessListsUpdateTimeMax=1.0000
fProcessListsUpdateTimeMin=0.2500
iLowProcessingMilliseconds=2
(These tweaks are experimental but for me it works, I don't know what it does exactly but I noticed periodic 1.5s spikes of -10 FPS in the city, like it's processing at the cost of stuttering the game. So - in theory, I cut down the time the engine dedicates to processing in between frames, letting it trigger at a minimum of 22.5 instead of 20 FPS, and allowed it to only spike for 1 second max instead of 1.5.
For me it helps smoothen out the lag spikes in the city, though it will still spike if running fast (you may encounter an unloaded cell that magically renders a few feet ahead of you for eg, but it helps it to be smooth enough to aim during heavy battle)
Updating the drivers and putting my CPU on max performance power mode as well as these tweaks has allowed me to play without VATs. All without overclocking.
But yeah, hopefully there will be mods or a patch to fix the performance issues in this game. Bethesda did release a patch back in Skyrim that overhauled/optimised the CPU usage of the game, so I hope they do the same here. If not the modders will do it in time. Or we can upgrade to a top-end machine, which is what I'm doing eventually.
BTW have you guys seen Star Wars yet?