Last year I switched from spinning around 90-100 rpm to a more old school cadence of 60-75 rpm.
I no longer have the cardio for spinning to be efficient. I'm not racing consecutive days for three weeks. I'm not taking EPO, or blood doping. All the stuff that makes spinning more efficient for younger athletes in peak condition.
Slower cadence works for me. My heart rate stays lower. My legs are stronger. No injuries -- I stay within a reasonable effort. I'm not trying to grind a 52/13 gear up a climb. Depending on the bike my easiest climbing gear may be 42/28 or 39/28. Usually I'm in the 24 or 25 rear cog, even if I'm tired. But the 28 is nice to have after long, hot rides.
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See this 2019 article .
Ideal cycling cadence: new evidence why amateurs shouldn’t pedal like Chris Froome
Scientists reveal more evidence of how pedalling a low gear at a high cadence could waste a cyclist's energy
"New experiments from the UK-Japan research collaboration have revealed one of the factors which makes that kind of pedalling wasteful.
“When the cyclists pedalled close to the ‘ventilatory threshold’ at 90rpm we were able to see the oxygen level in the thigh muscles had fallen, compared with lower cadences at the same exercise intensity,” says Formenti. “Participants’ heart rates increased by 15 per cent, yet their exercise efficiency actually decreased.”