In my dojo we have some black belts who got there in 5 years and kind of disappoint you when you practice with them, and we have 2 black belts who took about 15 years to get to their shodan because they started over in several different dojos (one of them got to 1st kyu in two different places), and they clearly kick arse.
I understand your frustration with these delays, but take solace in the fact that when you do get your rank, you'll have more experience and diverse knowledge than your co-ranked peers
One final thought - Is prior, similar knowledge a hindrance when starting in a new place? If so, I can understand how holding back a student with previous knowledge, even throwing them back to the lowest rank, makes sense by making them remember that they should approach the new system humbly and with a clear mind.