This happened to me last month flying UA from DFW to DEN. I was in zone 4 boarding (aisle seat). The GA came on the intercom and said anyone in zone 4 and had a roller board, they had to gate check their suitcase. I was hesitant at first because I knew once I landed at DEN, I would have to wait longer at the baggage claim. When I finally boarded (I was in seat 23D I think), there was plenty of room in the overheads for several more rollerboards. When we deplaned and I got to the baggage claim, my bag was already on the carousel. I was totally amazed. It was the fastest I had seen bags come out. I would guess it was a total of 20 mins from the time we parked at the gate on the B concourse (and we deplaned) that I was at baggage claim.