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Is your speed based on a GPS reading? WAter speedometers are notoriously inaccurate.

Assuming you have adjusted the trim for best performance the first thing I would do to further improve performance is raise the engine at least high enough that the plate is not below the keel. Then I would check to see if the plate is below water while the boat is on plane. If the plate is below water I would try raising the engine another notch and repeat the process until the plate is running right at water level or very slightly below. This is the least cost first step.

A second step is to ensure your engine is in good tune.

If the engine mounting height, trim angle and tune are correct and the RPM remains low I would seek advice on a propeller change. Folks will need your propeller information, diameter, pitch and anything else you know.