Fred DeLong reports that a beginning cyclist has to breathe twice as hard as an experienced one to maintain the same pace through traffic. Pedalling well involves balancing your leg mass vs the crank geometry, timing the muscles for optimum rates of contraction without knee damage (slow) or control errors (high.) Body position matters, too. It only looks simple.
I have a handy little card under my speedo that shows the RPM in each gear for a range of speeds. It's very easy to work out with a calculator.