>>why are these kids “finished” in half an hour? Because they are supposed to be learning what is on the maareh mekomos sheet.<<
If you are learning only for shiur, then you are correct. But if you are learning for the sake of learning, and you accept that Torah is Ikar, than you could easily spend the rest of time going deeper into the sugya, reviewing the previous shiur, going into a previous sugya which the shiur skipped.
Just because they are supposed to be learning what is on the ma’arei makom sheet, doesn’t mean they can’t learn anything else. They could also discuss with others how they learned the ma’arei mikomos.
The problem here is the mentality, not the ma’arei mikomos sheet. I have seen the ones who struggle in the beginning who later become greater than the “quick” learners. And do you know what? Even after they overtake them they don’t “give up” after the 30 minutes. Torah is valuable to them and they don’t wasted it (nor do they waste their time)