The following command removes the current directory of the path to display relative paths

cmd.exe /q/V:ON/c "for /f %A in ('dir /s/b/a:-d-h 2^> nul') DO set RELPATH=%A & echo !RELPATH:%cd%\=!"

/q is for @echo off
/v:ON is to enable variable expansion in runtime

If there are no files, an error appears so redirection to nul is required.

#971 (comment) is necessary to omit network drive for the preview. /a:-d-h is optional.

@junegunn Can you test if it works for you? I compiled using mingw32-make.exe from http://tdm-gcc.tdragon.net/download and preview works for me because the network drive is removed.

You can test it out in fzf.vim after forcing forward slashes (or escaping the backslash) on the filepath of the preview script
Reference: janlazo/fzf.vim@3657fc6#diff-7274b7c6c87526542ef9087f2e997dfaR38

Edit:
I mixed it up with FZF_DEFAULT_COMMAND.

I give on this.
For loop begins after dir is done so it is slow in large projects.
There's no native windows solution I can think of that has comparable speed to dir, can be configured and piped, and won't slow down the system.

I see, thanks for looking into it.

There's no native windows solution

Many users of fzf on *nix use ag or rg instead of find as the default command, if there are equivalent solutions on Windows, we can suggest those to the users.

From my previous note about powershell for relative paths, I recommend rg because of UTF-16 support and sift because only the prebuilt binary is required.

It would be great if you could mention your findings here: https://github.com/junegunn/fzf/wiki/Windows