Ruiz has some of the fastest hands in the heavyweight division. He's an excellent boxer with a unique skill set for which it's really hard to prepare.
Joshua will have more time to do just that in the rematch. For the previous fight, Ruiz came in as a late replacement for Jarrell Miller, who got suspended by the New York State boxing commission for failing PED drug tests. So Ruiz took the fight with just about a month left in the promotion.
That only left Joshua about that same amount of time to prepare for Ruiz's fast and heavy hands.
But it's truly hard to prepare for that type of opponent. Not only does he throw fast punches, but they are also very powerful. Sparring partners that can mimic that are hard to find, so Joshua probably won't have much luck finding an adequate way to prepare for Ruiz the second time around either.
The result could end up the same no matter how much time Joshua has this time.