FileNames becomes extremely slow when there are large number of files under a directory

I have a directory on my portable hard drive, with around 350000 files under it (there is no subdirectory, only files). I want to get all the file dirs so use

 filedirs=FileNames["*",mydir]

However, the code above run for more than 1 hour and still not return a result. Why is FileNames so slow? What should I do?

I’m using Mathematica 12.0 on my Win10.

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The problem is bypassed by running tree dir /f /a >outputtxt on win10’s cmd window, as suggested by @mikado. The size of outputtxt is only ~20M and it takes only several minites to get result. I still don’t understand why FileNames will take astonishing time to run.

filenamesFast[dir_]:=
Module[{txtname="\temp-alldirs.txt"},
Run["dir ""<>dir<>"" /b >""<>dir<>txtname<>"""];
(dir<>#)&/@StringSplit[Import[dir<>txtname],"n"]
]