The level of efficiency sought is to be adapted according to the Circuit Board Manufacturers. Regarding a recreational drone or used to take photos, you can aim for an efficiency of 7. However, for a racing drone, which will be small, agile and responsive, capable of acrobatics, it will be better to mount engines that turn fast. These engines have a high KV.
Note: KV is an important measure of the number of engine rotations per minute for 1 volt. For example, a 500 kv motor powered by a 3S motor (11.1v) will run at 500 x 11.1 = 5,550 rpm. The adequacy between the engine and its equipment. For a good efficiency, it is necessary to adapt the motor, the ESC and the propeller which work in a homogeneous way.
It is therefore a question of choosing a set of components that work well together. It is for this reason that it is advisable to start by buying kits or known configurations. This allows for better efficiency.