diff --git a/core/includes/file.inc b/core/includes/file.inc
index bb537fb..32a3e88 100644
--- a/core/includes/file.inc
+++ b/core/includes/file.inc
@@ -1099,6 +1099,10 @@ function file_unmanaged_save_data($data, $destination = NULL, $replace = FILE_EX
  * Directories and files beginning with a period are excluded; this
  * prevents hidden files and directories (such as SVN working directories)
  * from being scanned.
+ * 
+ * Scanning the entire public files directory is not recommended as it can
+ * create performance issues. Module authors should use their own subdirectory
+ * for their module related files or preferably use Managed files API.
  *
  * @param $dir
  *   The base directory or URI to scan, without trailing slash.