I thought planes like the 737 were optimized for 36k ft or so. Even if it's not flying right over the water at under 1000 ft like the P-3s do, I still would figure it's not going to climb that high between buoy drops. That means what, 10k ft or so? As mentioned, you can't shut off any engines when you only have 2, which the P-3s and others used to do to when you don't need to go fast. So while the normal range of a 737 far outstrips a P-3, it's going to burn a lot more fuel with climbing and descending and operating at non-optimal altitudes?
The slow part is because if you find something and need to turn around and drop buoys in a certain area, well, airliners aren't known for great agility.