If you're downloading a playlist you could run it in parallel to achieve your max network bandwidth speed:

  function ytp() { youtube-dl --get-id "$1" | xargs -I '{}' -P 200 youtube-dl -i --embed-thumbnail --add-metadata -f 'bestaudio[ext=m4a]' -o '%(title)s.%(ext)s' 'https://youtube.com/watch?v={}'; }