Why don’t you just ask your friend to try the theory? It’s a quick and easy manouvre to move a few thousand folders/files to a differetn location on the NAS.
You are a bit ambigous about whether you are talking about 10K files, in the same folder or 10K folders in the same root. Or just the summed amount of folders?
Still, its a lot easier to manage a pretty large collection if you have some strategy regarding the storage. I split my albums into grouped folders (i.e. ABC, DEF ect) and then into artist names:
