Let us imagine a universe in which, during the days of Adam, the speed of light was a million times faster than it is today. Now let us conduct a fantastic thought experiment. We get a couple of angels . . . Gabriel and Michael . . . to take us back in time to the days of Adam, along with a pocket mirror and a laser pointer. Now we shine the laser pointer for exactly one second, blinking it with our thumb twice in the one second, up into the night sky. This creates a long bar of light with two gaps in it. How long is the bar? one million times 186,282 miles. (Since the speed of light is, today, 186,282 miles per second). Gabriel flies alongside the light and keeps it squeezed together without changing its speed. Now Michael takes us miraculously to the exact time and space in the universe where he can place the mirror and reflect the beam back to the earth. He holds it there long enough to reflect the whole beam back, however long that is.
Now Michael takes us back again to our own time and we simply look up into the sky to see the beam coming back. We look up and see it. How long will the beam shine in our eye? Well, it is still the same length . . . one million times 186,282 miles . . . but now the light is crawling along at only 186,282 miles per second, so it shines for a million seconds. Those two gaps we set into a single second? It now looks as if our thumb had to take a million seconds to press the button those two times for those gaps.
The point is to make it clear that if light slows on its way to us, the events taking place we see by that light will be slowed. Things like the timing of cepheid variables, the decay of radioactive isotopes from supernova explosions, the rotation rates of the galaxies . . . these will all be slowed, in appearance, by slowed light.
Such slowing is not observed. The idea that light once traveled faster is ruled out by such observations.