It's not exactly easy but here you go:
There are three elements that cause excess constriction inside the exhaust. The first is carbon build up, which can often be cleared by driving consistently in a lower gear than normal, and preferably over long journeys. With mild steel exhausts, the second is rust, which is a curse, particularly with petrol engines used mainly for short journeys. And finally there's deterioration of the catalytic converter, with a variety of causes.
A deteriorating exhaust will increase fuel consumption, and the engine will have less power, particularly at low RPM.