You might try myadir() function in downloads ID # 10070 ADIR() - Extended Function.

It works like adir() function (in fact uses adir()) but can list files in subdirectories as well as specified directory. It returns the number of files found and create a cursor or an array depending on how it is called with same structure as adir() array with an additional field for full path.
On my network drive I tested it by listing the entire drive (over 70,000 files and over 21 GB) and it processed the list in less than 3 minutes. Not instantaneous, but not too bad.