At Z ... slowest speed
At Y ... fastest speed
At X ... medium speed
Wherever it is in its orbit, the line from the planet to the Sun smears over the same amount of area every second.
That's Kepler's second law of planetary motion.
The reason this happens is: That's how gravity works. (A better explanation is available, but first you have to be able to twirl calculus and solid geometry in the air on long sticks.)
