I read an article a while back about a person's efficiency at their natural cadence and then at some other one the rider had to maintain. As I recall the riders performed best at their natural spin, but these were trained athletes that spin fast anyway.

I started out slow (I actually developed tendinitis at that stage) and eventually got faster and faster. At a casual pace on the flats I like to stay over 100, at speed I like 95-100, this is natural and only discovered after I bought a bike computer. Climbing is a free for all because I still suck at it.