I've got a base Macbook model on default resolution, but I'm getting beachballs occasionally with 1080p video. My 2013 11" air plays with no lag, when video is tested after the Macbook 12 gets beachballs.
Could other users chime in with their results? Make sure it's set to 1080p youtube. Try changing from short, then long videos.
Could other users chime in with their results? Make sure it's set to 1080p youtube. Try changing from short, then long videos.