10 or 15 pounds is a pretty big number if you're talking about steering, accelerating, climbing, etc.

On the wind resistance front, it's more complicated than a flat 75% rate. Wind resistance actually increases with the square of your speed. It sounds like you have a pretty classic "postman" posture, maybe slightly less upright.

I'm out of town right now so I don't have access to it, but Bicycling Science covers it pretty thoroughly. Anyhow, if your wind resistance is x at 12mph, when you hit 24mph, it's going to be (24/12)**2 or 4 times as great. I do recall that even at relatively low speed, a postman position wastes a significant amount of energy.