The speed of light in vacuum is 299,792,458 meters per second,
and that's not an average. That's what it always is.
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About 2/3 its speed in a vacuum.
The speed of light is a constant, C= 300,000 km per second.
3/4th
There are several different types of glass, and light has a somewhat different speed in each of them. On the average, the speed of light across all typical types of glass is in the neighborhood of 2/3 its speed in vacuum, or around 200,000 km/sec .
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.