The real problem
NEF is not one format but a family that evolved with every camera generation. Older files sometimes show up in Windows, newer ones often do not. To you that looks random: one folder shows images, the next shows grey icons. On top of that, Windows only starts working once the folder is already open.
What the preloader does differently
It treats every NEF and NRW file the same and prepares them before you need them. The result is a library where every file has an image, regardless of which camera generation produced it.
- The same display for old and new Nikon files.
- Freshly imported cards are ready before you start culling.
- External drives are included in every plan.
Culling gets fast again
The real gain shows up while sorting. When every file appears as an image immediately, you decide in Explorer instead of opening one file after another in a viewer.