"Mercury's much higher orbital velocity"? Mercury moves more slowly than the Earth, just as Earth moves more slowly than Mars. The faster something moves around the parent body, the greater the distance.
http://www.windows.ucar.edu/glossary/orbital_velocity.html
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How fast a planet travels as it orbits around the Sun is called orbital velocity.
Planets that are close to the Sun have high orbital velocities because of Kepler’s second law of planetary motion. This means that Mercury, the closest planet to the Sun, is the fastest moving planet in our solar system moving at about 48 km/sec! Earth is orbiting slightly more slowly at a speed of about 30 km/sec and poor Pluto, far from the Sun, travels less than 5 km/sec.