A camshaft closes and opens the intake and exhaust valves of a car engine as the piston pumps up and down. The shape of the camshaft determines engine performance while a car goes at different speeds. With an engine running at high revolutions per second, the camshaft's performance degrades.
The efficiency of an engine is lower at high camshaft cycles because less fuel-air mixture is passing through the system. When a car moves faster, the intake and exhaust valves open and close faster under the direction of the pumping camshaft. This reduces the amount of time available for more air and fuel to make it through.