This is deliberate in 5th edition D&D. If look at a graph of the leveling rate from 1-20 it is really fast at levels 1-5 and levels 16-20, but slows from 6-15. Traditionally a lot of people have felt they have the most fun at levels 5 to 14 or so. As such the current leveling scheme stretches the time you are in those levels. Levels 1-4 are basically so fast that you could see them as the tutorial. For example, you're assumed to hit level 2 at the end of your first session assuming 4 hours.