Optimal cadence like many things in life, is a trade off between two things: muscular effort and cardiovascular effort. Riders who prefer a big gear (e.g. higher gear inches) have accelerate slower from a standing start and all other things being equal will have a lower cruising cadence. Pedal effort per stroke will be higher and the muscles will tend to fatigue faster than faster cadence. It is this increased effort per pedal stroke that can harm the knees in the long run.

Riders who prefer spinning on a smaller gear (e.g. lower gear inches and cadences from 100-120 rpm) typically accelerate faster from a standing start, have a higher cadence and correspondingly lower effort per pedal stroke. This makes it easier on the joints but more taxing on the cardiovascular system.

Higher cadences allow slightly faster in-gear acceleration which is important to TdF racers and criterium racers making sure they keep up with someone trying a breakaway. If you wish to take a precautionary approach in avoiding knee injuries, learning to spin comfortably at higher cadences is a good choice.

Having said that, there is not one optimal cadence for every rider. Keep a cadence which you think maximizes your power and endurance.

For me, my comfortable cadence range is from 85-105. Somewhere in there depending on prevailing wind. I ride a singlespeed between 62 gear inches to 73 gear inches depending on my bike of the day. At that cadence range, I am cruising around 30-34 km/h on the flats which is as much as I can do riding solo carrying commuter weight. Beyond that wind resistance is strong and I am not able to sustain such pace without tiring rapidly.