The thing is though that this behaviour is limited to these specific files.
Do you see the same slowdown if you use a file manager other than
Windows File Explorer? Such as Total Commander, Multi Commander, etc.
In my experience Windows File explorer is *very* slow in many operations.
Opening a folder with 700+ mp4 files can take 20-40 secs especially
if I am scrolling the list. Or changing the sort order, etc.
Among other things, Explorer apparently examines the contents of each
file and produces summary info - which can be seen when hovering the
mouse cursor over the file in the list. Details such as File Type,
Length (run time in mins and secs), etc.
By contrast, showing and scrolling the folder contents is immediate
when using one of the utilities mentioned above.