Most modern commercial air travel is actually slightly slower on average than it was in the 1960s. Cruising speeds for today's airlines are about 500 knots (575 miles per hour). However, 525 knots (600 miles per hour) was the specified cruising speed for the Boeing 707, which was the first commercially successful jetliner.
Modern aircraft are not designed to go much faster than this because an increase of speed burns too much expensive fuel. Additionally, at 667 knots (770 miles per hour) an airplane would hit supersonic speed, resulting in a sonic boom. While there have been commercial flights which deliberately breached supersonic speeds, many countries have restricted the use of this in their airspace, making it impractical for the majority of airlines.
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