It's not difficult to understand, to get more performance you chuck more fuel in the engine, to get more efficiency you chuck less fuel in
The two are pretty well direct opposites -
This is one of the most pernicious urban myths:
1) increasing power has to hurt fuel economy
2) disconnecting the emission controls will increase power and mileage
As for "power and efficiency" being opposites:
open your eyes folks. Look at the engines from the 60's and 70's compared to now. The 220 HP 318 CID engine in my 67 Dodge got 12 mpg in the city. My buddy just bought a new 5 Liter Mustang with 300 HP that gets about 20 mpg city and 30 mpg highway.
It is true that an idiot trying to "soup up" a well designed engine will likely cause a drop in fuel mileage since:
a) he has no clue what he's doing
b) he's following the "stuffing in more fuel will make it go faster" approach
but that certainly does not prove that power and mileage have to be at odds. In reality, a lot of the power gains made were done by reducing wasted fuel usage and improving efficiency of combustion.
My point is to warn people that screwing with their engines does not automatically increase power or mileage. Chances are it will increase emissions, shorten engine life, and damage components like the cat converter if it is fed an overly rich mixture for long.