does the LN/WN explain why those 2 grow so fast in comparison to the other people/"hero party" ? I can't remember that the manga expicitly stated why the two are so OP.
My guess,
If this is like some mmorpg there are bonuses for first times(first time defeating a boss or clearing a floor for example). (implied on the chapter where they defeated the floor boss but didn't get the drop)
Since it is their basement they can use it more(like those who have personal gyms are more efficient) and since they are the only ones using it they will get to defeat rare monsters that give bonuses.
There is also signs that they are strong even before the dungeon since well they are poor and living in a remote place. In the first chapter the guy bought supplies from the nearest supermarket which is 10 km so he have walk/run 20 km more if it is not a straight line. the girl too must be walking that distance everyday. That is half a distance of a marathon(42.195 km) and that is without carrying anything.
Therefore, they have stamina to spend more time at the dungeon.