c is a constant. That's not a property of light, that's a property of spacetime. It's a matter of geometry... the equal angle between space and time corresponds to c. You can think of it as a tangent a/b, distance/time. The angle always appears the same, to every observer. It's probably an inevitable artifact of observation... something about the way observation is structured, maybe. But, it's not something subject to variation. We do know enough about spacetime to understand that.

That's the whole magnificently cool thing about Relativity is that c is constant. That's why Einstein didn't like the name, "relativity", because the amazing thing he found out was that c is constant. That was the whole thing. Space and time have to adjust themselves to accommodate the fact but it's a fact for all observers at all times and has fascinating implications.

And, personally I would like to get the definition of "c" corrected to not be "the speed of light in a vacuum" but "the maximum speed at which information and energy can pass through space and equal to the constant velocity of all matter and energy through spacetime". Because, everything is always going through spacetime at c. The inertial observer's frame of reference moves along his/her time axis (world line) at the speed, c.

For inertial observers, the observer's time axis and world line are always coincident. For accelerating observers, their world line tips off-axis and they experience acceleration.

In 4-dimensional spacetime, everything always goes at c. Every particle with or without mass is moving at c along some trajectory in spacetime. That's why objects, like photons, moving at c though space cannot experience time... their total velocity is through space and the velocity component along the time axis is zero. The amount that time is slowed by velocity or acceleration are a matter of geometry... measured from the perspective of the observer. There is no universal or godlike perspective. Everything is relative to the observer's frame of reference. The amount of time slowing one inertial observer would see in a passing clock tower would be proportional to the cosine between the observer's world-line and the clock tower's time axis.