No. There are many reasons why it isn't free to just fly the airplanes slower, all have to do with the opportunity cost of time.
If an airline now flies passengers 20,000,000 miles a day (100 flights x 200 passengers a plane x 1000 miles a flight) and needs 50 planes to do it, cutting the speed at which those planes fly in half will mean that the airlines need twice as many planes. The fixed costs on the planes are the same whether they are going 500Mph, 300Mph, or sitting on the ground, either way the company is still paying the lease or mortgage on the plane. Flight attendants and pilots have to be paid regardless of how fast the plane is moving.
Additionally, for passengers, arriving later isn't merely an annoyance to be endured, it will change decisions. People have alternative uses of their time, mechanics can't fix cars when they are in the air, doctors can't save lives. The (possible?) reduction in ticket cost wouldn't adequately compensate some people for the additional time that they lose. What your proposal just did was to take a private decision (for the airlines - how fast do I want to fly my planes, for the consumer - which ticket do I want to buy from a market of airlines that compete to get me there the fastest, cheapest, or some combination) and have taken that choice away by making a government mandate.