In the late 1990s and early 2000s, experts established approximately 90 RPM as the perfect tempo. The reason is simple, logical, and still valid today.
At a lower cadence, you pedal with more force. Since your heart beats more slowly, this type of effort puts less strain on your cardiovascular system than pedaling faster to achieve the same power output. For example, 75 RPM to reach 250 watts, rather than 90 RPM to achieve the same result.
But it also causes muscle fatigue, which ultimately wears down our endurance.
Between the high crank speed seen in track cyclists (above 100 RPM) and the old fashion of large gear ratios, inherited from the days when cyclists and racers tackled monster mountain passes with a small 42-tooth chainring and a 24-tooth sprocket, sports scientists have established that 90 RPM is the ideal number, as it balances the effort between muscular and cardiovascular exertion.