Quick one (for me).

I play Fallout 4 at 1440p (2K) loaded down with mods -- not a nuclear desert -- rather a verdant, thick 3D modeled forest of 3D trees loaded with 3D weeds and grasses (as would be the case 200 years after the war). The graphical textures have been upped from 512 and 1024 pixels to mostly 2048 and 4096 pixels. A real load on the CPU and GPU.

I was happy that my new RX Vega 64 seemed to boost FPS significantly vs my Reference RX480. However, there were still sections of the cities (also overgrown with plant life) where the minimum FPS still dropped to 15 in some view directions (playable but too noticeable). It was virtually identical with my RX480 GPU. Obviously, more GPU power would not make much difference. And, CPUs don't get much faster than mine.

Today, I searched to see if there was an "answer". There are (by my count) 22 optional graphics settings in FO4. Normally, I have them all on the maximum settings -- usually called "ultra" (I do that in all games).

The answer: Only one mattered -- "Shadow Distance". I had that on Ultra. Setting to High or Medium doubled the slow parts from roughly 15FPS to roughly 30FPS (kept all else on Ultra). Many other sections away from the city have moved to near 60FPS or over (I use the Steam overlay FPS counter, my monitor refreshes at 60FPS so that's the max for my system).

Interesting to me that only one innocuous setting caused the issue (I assume shadows multiplied when the 3D models in view multiplied). Apparently, its not strictly a GPU issue. So, a faster GPU is not always the answer. FWIW, on my system, normal, unmodified games all run fast so far -- but, I like to self-mod and download mods -- its half the fun.