Top reasons are:
Exhaust Restrictions Fuel Injectors Fuel Pumps Worn Piston Rings Carbon Deposits On Intake Valves/Valve Seats Carbon Deposits On The Piston Fouled Spark Plugs
Dirty Air Filter
A clogged air filter makes the engine work harder to pull in air, and can restrict the maximum amount of air let in. Less air means less power.
The engine will have to work harder to push out exhaust gases, limiting power. Catalytic converters can clog up on engines with an improper air/fuel mixture or as a result of fuel additives reacting within. Mufflers can also fail internally, and alterations of the internal piping can result in an airflow restriction.
Deposits can build up on the injectors over time from heat soak or poor fuel. Small restrictions can cause the O2 sensors to read a lean mixture, and so more fuel will be added to compensate. This can result in a rich mixture for the cylinders without injector problems, or even misfiring if the injector isn’t capable of injecting enough fuel.
Overtime the fuel pump can wear out, but it might not necessarily fail catastrophically. While it may still be able to supply fuel at lower pressures, it may begin to struggle to provide fuel at higher pressures or for longer durations.
One of the major things that can happen over time is the piston rings will wear down, and this will allow for blow-by. Some of the high pressure air and fuel mixture combusting will pass by the pistons and travel along the cylinder walls into the crank case. This is pressure that should be pressing the piston down, so power is lost. It also means less compression as some of the air can escape as the piston travels upward on the intake stroke.
If carbon deposits build up on the valves or valve seats, this can prevent the valves from closing properly. If an intake valve cannot fully close, it will allow air to escape during the compression stroke, effectively lowering the compression ratio.
If deposits build on the piston or cylinder walls, these deposits can create hot spots. These hot spots can result in engine knock if conditions allow for it. If the engine is capable, it will retard the ignition timing to reduce the likelihood of knock. By retarding the ignition timing, power is lost.
Spark plugs can build deposits with time. Inconsistent spark plug firing means you’re likely to misfire. Maintaining clean plugs ensures that the spark part of the equation doesn’t result in a loss of power.
Worn Piston Rings
Carbon Deposits On Intake Valves/Valve Seats
Carbon Deposits On The Piston
Fouled Spark Plugs