How does move speed scale?

I just rewatched a game to verify this information. please explain it to me:

Thresh with 440 movement speed got from spawn to bot lane 3rd turret in 19 seconds. Janna with 401 movement speed got from spawn to bot lane 3rd turret in 28 seconds.

How does a 10% increase of move speed stat increase move speed by almost 30%?? Did I witness someone cheating with a speed hack?