Planetary orbits are rarely circular...they are elliptical...meaning that they do not orbit the source of gravity (e.g., the Sun) at an equal distance throughout their periods (e.g., year) of the orbit.
As a planet draws closer to its source of gravity it gains kinetic energy, just like when you drop a ball to the ground. And just like that ball, the planet speeds up with the increased KE.
Conversely, as the planet passes its nearest point to its star and moves on, it pulls away from its source of gravitational pull. In which case all that added KE is converted to potential energy and the planet slows down. When it reaches its farthest distance from its star, it begins once again to fall inward and pick up kinetic energy and speed.
It does this each orbit...speed up while falling closer to the star it's orbiting and slow down when pulling away from it. And there you have it "variable planetary speed." Actually, variable planetary velocity is a better, more meaningful term.