It just feels like we are seeing different players, I don't see speed as a big asset at all with Januzaj, to me his control and passing will be what allows him to hurt teams centrally like Silva, but he's not going to go past players and burn them off like Hazard or Sanchez as more often than not most have the speed to recover if he doesn't release the ball quick enough.
I wouldn't describe it as a big asset either, but nor would I consider it a weakness, and I don't remember ever thinking that it prevented him from escaping the attention of a man he'd beaten with his quick feet and tricks.
He certainly doesn't 'burn defenders off' like Sanchez; his pace isn't anywhere in that league. But I don't think Hazard does that either, because his pace, similarly, is decent rather than special. Hazard ensures he stays past defenders once he's beaten them through a dribbling style which really glues the ball to his feet, and his excellent balance and strength; he doesn't offer anyone else a glimpse of the ball, and certainly not a defender who is already behind him.
Januzaj, for his part, tends to escape cleanly because the
way he beats defenders - with very pure trickery and unplayably fast feet - tends to
really beat them. He'll put his man on the ground, or send him so decisively the wrong way that by the time he's got himself sorted out Januzaj is long gone. He only normally has to beat his man once.