What is typically a sign of a clogged fuel filter?

A clogged fuel filter restricts the flow of fuel to the engine, which can significantly affect performance. When the filter becomes blocked, the engine does not receive the adequate amount of fuel it needs for optimal operation. This results in decreased power output as the engine struggles to perform under insufficient fuel conditions. Drivers often notice a lack of acceleration and overall sluggishness in the vehicle's performance due to this restriction.

The other options do not align with the impact of a clogged fuel filter. Increased engine speed, smoother operation, and improved fuel efficiency are typically signs of a well-functioning fuel system, where the engine receives an adequate supply of fuel and operates efficiently. Therefore, decreased power output is the most accurate indicator of a clogged fuel filter.