title: Concat videos too slow using Python MoviePY
body: <p>I am joining 50s or more videos of 1,2,3 minutes using MoviePY but it's giving me 20 hours, even though I have 64 GB of ram, i7 and GTX 670, not top of range yet reasonable. Is there anyway I can speed this process up ?</p>
  
  <pre><code>padding = 10 # padding option
  video_clips = [VideoFileClip(video_dir + video) for video in os.listdir(video_dir)]
  video_fx_list = [video_clips[0]]
  
  idx = video_clips[0].duration - padding
  for video in video_clips[1:]:
  video_fx_list.append(video.set_start(idx).crossfadein(padding))
  idx += video.duration - padding
  
  final_video = CompositeVideoClip(video_fx_list)
  final_video.write_videofile(video_dir + 'myoutfile.mp4', fps=24)
  </code></pre>
  
  <p>I don't need original audio these clips have, would removing it going to speed things up ? not sure how to remove audio though/</p>
  
  <p><a href="https://i.stack.imgur.com/uPpqh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uPpqh.png" alt="enter image description here"></a></p>