Wait wat? Making an unknown number of people travel to a different city is a "quick" way to count them?

Hardly. It took Mary and Joseph about a week to go one-way to Bethlehem, and they only had to go 80 or 90 miles. A genuinely quick way would be to have people state their tribe while they're being counted. "But how do you know they're telling the truth?" Same way you know they traveled to the correct town to be counted, or that they didn't simply ignore the order and stay put. Having them move just to be counted is no more accurate than simply asking their tribe, definitely less quick, and generally makes no sense.