Here's a quick chemistry lesson about wood fires...
As wood burns in a campfire, flammable "wood gas" is released in the form of volatile organic compounds (smoke). These "wood gases" contain about 50% of the wood's potential chemical energy.
Here's the important thing to know... Wood burns at around 500° F, but the "wood gases" don't ignite until around 1100° F.
If your fire is not hot enough, this unburned fuel forms particulates in the form of smoke.
Smoke is wasted fuel.
So if you want an efficient burn, you need your fire to reach at least 1100° F, because this will ignite the "wood gas" and will release a lot more energy.