DL and B6 both have JFK-ORD flights departing at 0820, DL using a CRJ-900 and B6 an Embraer 190. The DL flight is scheduled to arrive ORD at 1034 and the B6 flight at 1003.
What accounts for the 31 minute faster B6 block time? Is their schedule unrealistic, meaning the flight is likely to arrive late? I don't think there's much difference in cruising speed between the CRJ-900 and E-190, especially on a short route like JFK-ORD (643 nm). The same thing affects two other DL and B6 flights on the same route later in the day (although not departing at the same time) where B6 block time is half an hour faster than DL.
I have also noted that DL has much longer block times (around 30-40 minutes) on LHR-JFK than other carriers flights departing at about the same time. While this probably ensures that DL flights dont arrive late, it makes them appear uncompetitive with other carriers in systems which often use fastest block time in prioritizing flights in displays. And it must reduce DL aircraft utilization somewhat compared to other carriers.