Of course airlines optimize it, but still, boarding alone takes a significant portion of the total time needed for a long-haul flight.
If I look at the latest long-haul flight I took, the flight time was 11 hours, turn-around time at airport 4 hours, for a total of 15 hours. If you'd drop the flight time to half, and manage to do accelerate boarding, cleaning, re-fueling etc in 3 hours, you'd come up with 8.5 hours. Much better, but not even close to be able to deliver twice as many flights per day. It'd be more like a 50 % improvement.
Long-haul utilization also depends on one other factor, which is time zones. It can be worth leaving the plane on the ground longer in order to line up for a more desirable departure/arrival time.
You see this a lot with transatlantic flights, where they spend more time than necessary on the ground at each end, but doing so sets up better timing (like eastbound TATL flights departing in North American evening and arriving in European morning).
If I look at the latest long-haul flight I took, the flight time was 11 hours, turn-around time at airport 4 hours, for a total of 15 hours. If you'd drop the flight time to half, and manage to do accelerate boarding, cleaning, re-fueling etc in 3 hours, you'd come up with 8.5 hours. Much better, but not even close to be able to deliver twice as many flights per day. It'd be more like a 50 % improvement.