Complex sentence: The answer given is that Śāriputra and Maudgalyāyana were just like kings, who required a longer time to prepare for a journey than commoners: their attainment was of greater depth than the other students and therefore required more time.
Split into simpler sentences: The answer given is that Śāriputra and Maudgalyāyana were like kings, who require a longer time to prepare for a journey than commoners. In other words, their attainment was of greater depth than the other students and therefore required more time.