For the first version of the game and some of the less extensive updates they used the common streaming system and just increasing the memory pool up to the maximum supported (3072?) could alleviate the worst of it.
After that from what I understand the game has a sort of grid map and passing through these unloads and loads data which is much harder to alleviate and the game isn’t built around newer tech to take advantage of additional CPU cores or a SSD.
There are benefits but only up to a point before the engine even modified as it is just hits it’s limits and once in the actual car you start hitting these zones rapidly or when gliding at a higher speed though from above the reduced LOD’s slightly help far as I recall but there’s still going to be hitching and stuttering.
Downgrading breaks all kinds of fixes and added/fixed effects plus later DLC support.
DXVK could work but on NVIDIA systems the NVAPI implementation could be a bit tricky for various effects and optimizations and disabling these will either break performance or some of the graphical effects these are used for.
(Getting some of the newer API advantages and features even if not in full one reason even on NVIDIA that DXVK can improve performance by a good margin or mitigate micro stuttering or framerate fluctuations.)
For SpecialK the texture cache might help somewhat but with these grids or zones it’s going to be loading assets anyway so the benefits with this if it works without other Unreal Engine texture issues might be limited as a result.
Capping at a lower framerate also helps but that’s not something I’d recommend the game is a lot better after that 30 FPS cap was removed going back is not optimal.
Don’t have any other good suggestions, console perhaps X Series with backwards compatibility that’s a bit of a suggestion but some of these ports are kinda bad or very tough to get to work without some issues or bottlenecks or other problems unfortunately.
WB was probably eager to drop it too when outsourcing the support only slightly mitigated the reports and poor reviews and by that point the peak sales period had also gone really could have done with more support but that’s true for a lot of their published games even from more accomplished PC development studios.
(Rocksteady I think was mostly doing the console builds, another studio did PC and then a third took over for when they started fixing it up from how I recall.)
EDIT: Bit speculative there at the end, another optimization patch and final QA and fixing up some of the issues would really have helped but it’s a bit of a miracle they stuck around for a while after that major update and the DLC were out to where the rain effect got fixed up before stopping support entirely.