Perfectly competitive firms are more efficient than monopolies because in a perfectly competitive market, firms have to accept the price prevailing in the market. So, they adjust their cost and revenue policy according to the prevailing price, thereby making them cost effective. One the other hand, a monopolist is a price-marker. He can formulate his own cost and revenue policy. So, there is no compulsion to control costs. So, firms under perfect, competition are more efficient that a monopoly firm.