Isaac Newton showed that the attractive force of gravity decreases the farther you are from an object. For example, the gravity of the Sun acts on both the Earth and the Moon. But because the Moon is closer to the Earth, the Moon is more attracted to the Earth.
As a result of this, you must remember two things about how planet speeds vary with distance:
1. More distant objects feel a weaker gravitational force, so move slower.
2. Distant object have further to go, so take more time to complete one orbit.