Emms needs to traverse directories in order to find playable
media. The default method Emms uses to achive this is
emms-source-file-directory-tree-internal as defined in
emms-source-file.el. The above method is written portably and
will always work, but might be too slow if we want to load several
hundred tracks (or more).
emms-source-file.el defines another method for finding files,
emms-source-file-directory-tree-find which uses
GNU/find. emms-source-file-directory-tree-find is usually an
order of magnitude faster, but of course will not work if you do not
have GNU/find installed.
The method Emms will use is defined in the customisable variable emms-source-file-directory-tree-function.