Every aircraft has an optimal cruise speed at different altitudes, temperatures, and other varying conditions.
When you fly above that optimal cruise speed, you lose fuel efficiency. When you fly below that optimal cruise speed, you will also lose fuel efficiency.
Many factors determine the aircraft's optimal cruise speed at the varying conditions, from the wing design, weight, etc.
Modern aircraft computers are capable of calculating the optimal cruising speed by taking into account the varying conditions, constantly following it and making any necessary adjustments.
I assume UA
and other airlines will start following optimal cruise speeds more often now.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran