chasm54

I disagree with the latter statement. The pros race >160kms per day without knee issues resulting from high cadences, and in my view the reduction of force through each individual pedal stroke will reduce wear and tear on the knees, not increase it.

There's no rule about this, some people favour a lower cadence, some a higher. But I don't think there is any dispute that the higher the cadence (up to the limits of what the cyclist can readily manage), the easier it is on the legs and the harder it is on the CV system. The higher cadences cost more in energy (hence the higher HR for a given power output) but generate less muscular fatigue. I would surmise that those doing long distances at a rather slower pace than pro racers will tend to default to a slightly slower cadence because:
a. they aren't putting out as much power most of the time, so their muscles cope just fine with the force required;
b. it represents the best compromise between power generation and energy conservation when spending a long time on the bike.

OP as you start learning to spin your speed will probably drop, yes, but only temporarily until you build up the leg speed and get used to routinely riding at a higher cadence. Like I say, if for a given speed you are usually in (say) the 53/13, change down to the 53/15 and seek to maintain the same speed. You'll find that it takes a surprisingly short time for you to be able to do this, and as you keep at it your "natural" cadence will slowly rise.