Economy tier books might play a role in the training of graders advancing past Modern, thus the longer wait times. I consider them to be the same entry tier, modern is just discounted so they can offer cost effective grading on newly released books. Economy really helps people who don't know the value of the golden/silver age books they might have just inherited or come across. Even if they get bumped up to a higher tier they may well have never been submitted in the first place if the submitters had been scared off by the higher costs of the faster tiers. Much nicer knowing those higher grading costs are justified than having to roll the dice, right?