Like others, I have experienced excruciatingly slow syncing times when syncing with LRm. Often syncing would stall altogether. I tried everything recommended in the many posts about this issue, but the stalling continued. Then I looked at the collection that was being synced and noticed that it was stalled on generating a smart preview of an externally edited tiff file. Once I removed this file from the collection being synced, syncing went ahead again until it encountered another externally edited tiff file it couldn't digest. (Files are sometimes edited by PS, Color Efex Pro or Photomatix Pro.) I understand this issue was supposed to have been dealt with in LR 5.5. Well, it hasn't.