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The progress of the party through dungeon levels evidently slowed down once they started going to stage 16 and above.
They weren't in the process of having Ren catch up to the party's overall progress anymore; they're now advancing and retreating together. Additionally, they are also spending time training in the dungeons, putting themselves at risk and honing their skills under pressure.
Since it doesn't matter how much time they spent inside the dungeons, when only 30 minutes will pass in real life upon their return, this is how they could squeeze more time in the day to advance their martial practice.
Of course, this isn't applicable every single time, but for the most part, many martial artists take advantage of this.
This is exactly what the Green Tea Party was doing right now.
They stayed on stage 16 for a whole week. They repeatedly cleared it even though they've grown more than strong enough to handle it.
