I mean when you already know all the plot points and how to clear the trickier missions. A lot of people struggle early on because they didn't know about the 220 senzus or using IT to navigate through HFIL. For instance, it probably took me around 20 hours to beat pokemon ruby the first time despite it not really being a long game, but I can probably clear it in half the time now that I've played that game many times.