var all_videos = document.getElementsByTagName("video");
var factor = 2; //The playback speed factor ('2' = twice as fast; '0.5' is half as fast)

for (var i = 0; i < all_videos.length; i++) {
  all_videos[i].id = "vid_no_" + i;
  document.getElementById("vid_no_" + i).playbackRate = factor;
}