We can measure a circuit's efficiency in two directions: space and speed.
The space factor relates to the fact that each transistor takes up space, and the chip containing the transistors is limited in size, so the number of transistors that fit onto a chip is limited by current technology. Since CPU designers want to fit many features onto the chip, they try to build their circuits with as few transistors as possible to accomplish the tasks needed. To reduce the number of transistors, they try to create circuits with few logic gates. Thus we can approximate the space usage of a circuit simply by counting how many logic gates the circuit includes.
The second factor, speed, relates to the fact that transistors take time to operate. Since designers want circuits to work as quickly as possible, they work to minimize the circuit depth, which is the maximum distance from any input through the circuit to an output. Consider, for example, the two dotted lines in the following circuit, which indicate two different paths from an input to an output in the circuit.
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