Just looking in Task Manager statistics for yor MSFS drive you could see that there
nothing near than high disc usage during loading into main menu of MSFS.
Upgrading from a SATA SSD (approx 350MB/s) to NVME SSD (approx 3500MB/s) for me
gave near to zero performance gain.
My guess: MSFS reads only the small JSON and XML files to parse the content to expect and build
the memory library and base world to work with during flights.
To validate this i tried adding 10 big airports (in sum same size on disc as fstltl lib) and measure time compared to the fsltl library with 1500 mini sceneries (every aircraft livery combination has its own xml/json files) … the 10 airports have minimal impact on loading time, the fsltl files double loading time.
The only time where i expect improvements i during load into flight and scenery loading during flights with less impact to fps.