Oliver I’m getting a very similar issue in a new project, retina MacBook Pro, and a FireWire 800 drive using either proxy or optimized media. Footage in the browser plays back fine and skimming is smooth for the most part. However if I go to play the sequence I get a 1-2 second delay if editing off the internal ssd and up to a 15 second delay if the media is on the external drive. I think it might have to do with some sort of preloading, as the delay gets longer the earlier in the sequence you start (although its only a minute long sequence). I’ve tried trashing preferences, using a different drive, and transcoding the media but nothing seems to help much. The lag makes the program extremely irritating to use.