Q. If the sun is disappearing to perspective, shouldn't it slow down as it approaches the horizon?
A. The sun moves constant speed into the horizon at sunset because it is at such a height that already beyond the apex of perspective lines. It has maximized the possible broadness of the lines of perspective in relation to the earth. It is intersecting the earth at a near 45 degree angle.
It's widely observable that overhead receding bodies move at a more constant pace into the horizon the higher they are.
For example, a flock of birds receding from the observer will reach the horizon line sooner than a jet airplane flying at 40,000 feet. Due to its height the jet airplane moves through the sky slower than the flock of birds close to the ground and will touch the horizon line later.
Or, to make a slightly better comparison, a flock of birds at 300 feet will appear to reach the horizon line sooner than a flock of birds at 10,000 feet, despite the two moving at the same speed.
When a body increases its altitude it broadens its perspective lines in relation to the earth and the observer, and thus appears to move slower and at a more constant pace into the horizon.
Th sun and stars are at such a great height that they have maximized the perspective lines, moving into the earth at a 45 degree angle in relation to the observer, at a constant or near constant pace.
Magnification of the Sun at Sunset, or how the sun maintains its size throughout the day.