Question

Why is Mercury's orbit so fast?

Answer 1

Johannes Kepler studied motion of planets and made tree laws regarding planetary motion.

Explanation:

THe third law states that "The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. This captures the relationship between the distance of planets from the Sun, and their orbital periods.".
in simple terms closer orbits will have higher speeds.
Quote Wikipedia.