Never get why people think Januzaj isn't quick? He prefers to use trickery and movement to get past players but when he's faced one on one, there's time where he just knocks it down the line and leaves the defenders for dead. He's definitely not slow, and has enough acceleration and top speed to get away from most defenders.
Around the box and in tight spaces, he obviously uses his great dribbling skills, trickery and close control to beat a man. He normally stands them up (similar to Nani) and drops a shoulder to go one way and then goes the other. If he gets a run of games this season, then I think he can flourish and finally start to show his ability. For me, he can develop into a terrific no.10, his passing while inconsistent at the moment is very good but he seems to be under pressure at the minute and trying too hard to make things happen.
From a team perspective it would make sense to put him on the right for now and Mata as the no.10 or Herrera, Mata slows the game down too much out wide and is too weak and slow to be effective there while Januzaj does have all the attributes to be a great winger who can cut in and play dangerous balls or go on the outside and beat his man.