Refresh or Reload page if the midi files get tied up. Also, remember that if you can't play the riff at the actual speed Hammet plays it at right away, don't give up. It's much more important to understand the riff and be able to apply the riff in actual musical situations. That means understanding where it comes from and where it can be used and being able to play it at many speeds. You Can get this to full speed.
Speed is simply repetition and lots of it.
The hard part is learning to use the correct fingers and the best picking pattern. Always be consistent with your fingering and picking patterns. Once you have this part down, dig out your metronome or buy one and get yourself a calendar. Each day play the riff at a tempo where you can play it at a least 5 times in a row without a mistake. Mark that tempo on your calendar.