The orbital speed of a planet is reflected in the geometry of its orbit. Put simply, a planet orbiting closer to the sun travels faster than a planet orbiting further from the sun. That's also true of a planet whose orbit takes it closer and further from the sun. Such a planet travels faster when close to the sun than it does when it's further away.

Although it's a bit more complicated because the sun and each planet orbit around each other, it's a good approximation to assume each planet orbits the sun. As a planet orbits the sun, it travels on a path that takes it from its closest approach at perihelion to its furthest approach at aphelion. The closer those two distances are to each other, the rounder the orbit, which means the orbital speed will vary the least.

Eccentricity is a measure of the "roundness" of an ellipse. An ellipse with an eccentricity of zero is a circle. If a planet had a perfectly circular orbit its speed would never vary, but no planetary orbit is a perfect circle. Earth's orbit has a small eccentricity, at 0.017, but that's only the third lowest in the solar system. Neptune is second lowest, with an eccentricity of 0.011. The planet with the lowest eccentricity is Venus, at 0.007. That means Venus has the most circular orbit of all the planets, which also means it has the smallest variation in orbital speed.