Originally Posted by Bob Ross
While I don't doubt your personal experiences, and I've never ridden a fixie, I'm having a really difficult time understanding how this could be. There's just no scientific explanation I can imagine to support the implication that the jockey wheels in a derailleur are sucking up 25% of the energy your pedals put out!
It's not the jockey wheels. It's the freewheel and the ability to coast. You stop pedalling, you coast....you loose your momentum much fast on a freewheel'd bike than on a fixed gear bike. Plus on a fixie your legs never stop moving, thus the blood keeps flowing and you don't get fatigued or tired as fast (once you get in good shape).
But if you've never ridden a fixie...it all seems counter intuitive. Google it. You'll find many training methods that used fixed gear bikes. Read Sheldon Brown's articles on fixed gear. Read track bike training articles to learn even more.