Question

why does an object moving fast appears moving slow if it is far away?

Answer 1

What our eye perceives as speed is actually the 'angular velocity'. If the angle of the body's position changes fast, it will appear to move fast.

The angular instantaneous angular velocity is given by: \[\omega = v/r\]
where v is the actual velocity of the body, and r is the body's distance from you.

As you can see from the equation, when the body is farther, its angular velocity is proportionally lesser. So it appears to move slow.