A planet's speed around the sun is slowest when it is farthest away from the sun. This is due to Kepler's Second Law of Planetary Motion, which states:

"A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time."

This means that a planet covers more area when it's closer to the sun and moves faster. Conversely, when it's farther away, it covers less area and moves slower to compensate.

Here's a simple analogy: imagine a figure skater spinning. When they pull their arms in close to their body, they spin faster. When they extend their arms out, they spin slower. The same principle applies to planets orbiting the sun.