The speed of light is a constant, C= 300,000 km per second.
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They are equal.
The speed of light is not limited in a vacuum - the speed of light is fastest in a vacuum. But that is what Einstein called the "Cosmic Speed Limit" - nothing can move faster than the speed of light in a vacuum, or even quite asfast.
If both of them are in the same substance, then their speeds are equal.
About 2/3 its speed in a vacuum.
The speed of light is 186,000 miles per second in a vacuum.