Here's why:

* Orbital Speed: A planet's speed is determined by its orbital velocity. Mercury is the closest planet to the Sun, and the closer a planet is to the Sun, the faster it needs to move to stay in orbit.

* Kepler's Laws: Kepler's Laws of Planetary Motion describe the relationships between planets' orbits. The third law states that the square of a planet's orbital period is proportional to the cube of its average distance from the Sun. This means that planets closer to the Sun have shorter orbital periods and therefore move faster.

While Mercury's average orbital speed is the fastest, its speed varies throughout its orbit. It moves fastest when it's closest to the Sun and slowest when it's farthest away.