One of the slowest transfers to ever occur, if it ever occurs is the Erik Palmer-Brown to Juventus transfer. There have been reports and rumors of it happening for a year or more but Sporting Kansas City CEO Robb Heineman made it seem imminent back in March. At that time he said it was 80-20 that it would happen.
While a deal could have been struck, Palmer-Brown could not have actually been transferred then. Under FIFA rules, article 19 states: International transfers of players are only permitted if the player is over the age of 18.