On one hand, not everything happens in FPS, on the other.... I have never checked FPS. All I can talk about is general responsiveness, the almost complete elimination of infinite loading screens and speed of texture pop-in and so on.... the latter two having nothing to do with FPS. And I am not about to do multiple installs to run experiments to prove anything, ur just gonna have to take my word for it.....FO4 has severe memory problems late game in crowded areas and a SSD helps with that for whatever reason.
I don't know why it works, I just know it works. I also know that getting a SSD is basically standard advise given on forums for FO4 to deal with IFS when the other methods don't work.... or at least it was years ago when last I searched.
EDIT: to be fair for some people apparently nothing works to deal with IFS.... I got lucky that a SSD solved the issue for me, and no this is something that was never patched out, probably a hard limit on the game engine that they went over is my guess.
You won't get a more frames period. Fallout has specific settings tailored to your system, this includes things like LOD , shadow distance, MSAA, SSAO, ect especially with regards to GPU and system memory. Using settings over and above what is recommended is going to eat away at the memory, which means the next set of assets needs to be loaded into memory of both the system and GPU ram.
While SSD would improve loading and swapping content to memory it is all it would do, with plenty of GPU memory or even just enough system ram should make light work of preloading content into memory, which while initial preload will be slower on a HDD it would make an SSD irrelevant for the most part with enough system resources.
Most issues with regards to fallout is driver and hardware related especially with pascal cards at the time.Mirco stuttering isn't a result of the lack of an SSD, as the games isn't being played on the HDD or SSD but from system and GPU memory.
Believe it or not, average SSD bandwidth is a theoretically 6 gigabits these days, additionally the SSD needs to share bandwidth with other motherboard hardware, while Sata allows for two way data transmission it does come with a performance hit, if it isn't enough that bandwidth needs to be shared with other things it needs to share bandwidth with read and write as well.
Consider the bandwidth of a 960gtx, the bandwidth between GPU and GPU memory is 112 gigabits, the actual bus bandwidth between the GPU and CPU is around 48gigabits, 256bit card 96gigabits and considering the GPU PCIe lane isn't being shared with the rest of the system with the exception of pcie slots and the pending the chipset in use.
Compared to the SSD's dismal theoretical bandwidth of 6gigabits and having to share bandwidth, it isn't even remotely capable of meeting those bandwidth requirements let alone, on the fly asset loading you are insisting games are doing.
Which is why game data when starting a new level is preloaded into memory, especially geometry and textures is loaded into GPU memory due to the extremely high bandwidth of GPU memory.
However not every thing is handled by the GPU, CPU deals with every thing else the GPU doesn't and this is delegated to between the CPU and system memory which also has memory bandwidth far better then what an SSD can muster.
Textures and geometry is handled exclusively by the GPU, however this still needs to be passed on from the Storage to the system ram, cpu and then GPU memory, where it is processed into a useable raw format for the GPU to use.
Considering a storage device isn't part of the rendering pipeline at all, the storage device is only able to pass on data to where it is needed more slowly or faster with regards to feeding data to the system, doesn't in any way decrease or increase performance based on storage in use.
Theoretically you could load an entire 5 gig game into the system memory or preload part of the data as games normally do via storage and the framerate will remain exactly the same, irrespective of the method used.
I hope this some how puts to bed any issues with regards to on the fly assets loading or apparent physical performance gains as a result of using an HDD or SSD.