Actually, spinning faster is less efficient, but efficiency isn't the most important thing. There are several reasons to spin faster than is efficient, long-term preservation of the knees being a big one. Knees don't last a lifetime, so being gentle to them helps prolong their time.

I've never had a cadence meter, but every so often, out of curiosity, I'll count my strokes and divide by time. I'm not sure, but I suspect I pedal at around 90 rpm.