The efficiency of a propeller (or jet engine, or whatever) is a measure of how much thrust is produced, compared with how much power was generated by the engine in order to generate that thrust. "We generated 90 thrust horsepower in the climb, by running the engine at its rated power of 180 horsepower. So in the climb, our propeller is 50% efficient."
Now: drag. Each individual propeller blade (or fan blade, or whatever) suffers drag as it whirls around through the air. If you want more efficiency at low speed, you'll get a larger diameter fan as Mark1 said. But then the tips of each blade are moving quite fast at the rated RPM. If you try to go fast through the air, increasing RPM to do so, your blade tips will suffer very high drag in a rotational sense, leaving them unable to turn fast enough to generate thrust. So engine efficiency decreases - same power in, less thrust out.
Note that the thrust is axial: in the line of flight. But the drag in this sense is tangential: in the plane of rotation of the fan. Don't fall into the trap of trying to compare axial forces (thrust of a propeller, drag if it's windmilling) with tangential forces (torque of the engine).
Hope that helps.
Last edited by Oktas8; 5th Feb 2013 at 08:36.