I think I get what you are thinking. I picture it this way. The ultimate speed of light would have to be found at a place where atoms of matter move the slowest in the universe.
the place with the weakest gravitational field.in other words , a sun in the furthest orbit of its galaxy that is traveling around the weakest not very dense center of a galaxy in the universe. If you took an atomic clock which operates by vibrating an atom 9 billion times by hitting it with a microwave(the definition of one second ) to this place .This would be the fastest 9 billion vibration (one second ) in the universe. At this place you would still measure light speed as you would any where in the universe but this frame of reference would be the fastest second in the universe.
their is a speed limit to light. its just that matter will always slow down in direct proportion to light.