Think pumping losses. Any restriction in the airflow is going to increase pumping losses. Grab your bycycle pump. Lay the hose loose on the ground and start pumping. Pretty easy, huh? Now connect it to a tire. Start pumping. A bit harder? Same amount of air is moving, but it takes more work to move that air. (yeah, I know, presure differential vs. simple reduced orifice but most folks don't have the stuff laying around to demonstrate it exactly). Same thing if you are moving a bunch of air through a restricted air filter. Work performed = fuel burned.