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Galileo
The fastest that anything can go is the speed of light in a vacuum.
The universal speed limit for material objects is c : the speed of light in a vacuum.
Gravity causes all objects to accelerate at the same rate in a vacuum. In air there is air resistance which can slow some objects down eg a parachute. So, yes, in a vacuum all objects reach the same speed in the same time period.
Galileo said any two objects in a vacuum will fall at the same rate.
It varies, depending on the material. Typical speeds are about 2/3 the speed of light in a vacuum. Search for a table of "index of refraction" for different materials. If a certain material has an index of refraction of 1.5, that means the speed of light in that material is 1.5 times slower than in a vacuum, so you calculate 300,000 km/sec divided by 1.5 = 200,000 km/sec (where the first speed is the speed of light in a vacuum, and the second is the speed of light in the particular material).
Light travels faster through a vacuum. Velocity does not change in a vacuum, so objects retain their original speed and direction unless acted upon by another force, like gravity.
All will fall at the same speed in vacuum because there will be no air resistance. The gravity will pull all objects in the same force.
Objects in a vacuum will NOT fall at a constant rate; they will fall faster and faster. In other words, they will continuously accelerate. The acceleration near the surface of the Earth is about 9.8 meters per square second. This is not a speed - it means that every second, the speed of the object increases by 9.8 meters per second.
About 2/3 its speed in a vacuum.
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.
In vacuum, the speed is 299,792,458 meters per second.