Power is proportional to the cube of speed. This is primarily due to wind resistance. So, it takes way more power to go from 20 to 21 MPH than it does to go from 4 to 5 mph. If your ride has the same amount of uphill as it does downhill (such as a round trip route), then you're better off putting your power into the slower speed segments (uphills).

But that rationale makes lots of assumptions. For example, it assumes that you have a fixed amount of total power (energy) to deliver during your ride, and that you can apply power at any rate. In reality, the amount of power you can provide depends on the rate it is provided (slow and steady wins the race), and there is obviously a limit to your peak power output.

So, when you're talking about the bike/body combination, maximum efficiency is more related to the relatively narrow range of efficient power production of your body. In other words, find the rate of power delivery that you can (or want to) sustain for the duration of your ride, and stay close to it, regardless of incline.