If your spark plugs are old and worn and haven't been replaced in 100,000 miles or more then replacing the spark plugs with new ones can increase the engine performance as well as horsepower by at least some.
Old worn spark plugs don't allow the fuel to ignite properly which can increase fuel consumption and cause the engine to lose power.
It's also a good idea to replace the spark plug wires along with the spark plugs to get the best increase in performance.
Spark Plugs really should be changed after 100,000 miles to keep your engine running smoothly and provide the needed horsepower.