A BC sized properly for the amount of weight you're carrying, and for your body size, will have less drag than a bigger one. The less weight you carry, the less air you have to put into your BC to maintain buoyancy, and the less bulk you have to drag through the water. Most divers can drop two pounds or so. And get trimmed properly, so that when you're neutral you're horizontal. If you have to kick yourself up to maintain your depth, you are wasting energy by carrying weight with your fins and by presenting a larger frontal area to the water.
Water resistance increases exponentially with speed.
Swimming twice as fast requires four times as much energy. So all your movements, not just swimming, should be in slow motion. Kick within your slipstream. Wider kicks increase drag. It used to be said that large, slow kicks were more efficient than short, fast ones, but this is not necessarily true.