Have you ever realised that now aeroplanes fly at slower speeds than they used to in 1960s? Specified cruising speeds for commercial airliners today range from 480 and 510 knots, compared to 525 knots for the Boeing 707. Why? The reason of this difference is more effective fuel consumption today.
The efficiency of modern aeroplane peaks are at lower speeds because of the fact that engines achieve the same thrust with more air at the lower speed by routing most of the air around the engine’s turbine instead of through it.
Information source: MIT School of Engineering