The tires on the 7.2 look like 32c tires so they aren't knobbly mountain bike tires but they also probably don't have the lowest rolling resistance. Combine that with a typical upright riding posture and it probably takes about 100 Watts to bike 12 mph. That's not that much. However, to go 15 mph it would take about 170 Watts as the power need to overcome wind resistance triples or so.
So... if you want to go faster you can save some watts with different tires and a whole lot of watts by getting more aero on your hybrid bike. You can easily cut your power requirements by 30-40% that way.