The speed of infrared light in a vacuum is the same as the speed of any other type of electromagnetic radiation in a vacuum. This speed in meters per second is approximately 3 times 10 to the power of 8 and is a fundamental constant of the universe, meaning it does not vary regardless of your frame of reference or location.
Although the speed of infrared light in a vacuum is always the same, its speed in various substances varies widely. As the electromagnetic waves travel through the substance, they interact with atoms and molecules. Often they are absorbed and re-radiated. The net effect is a decrease in speed.
Infrared light in air travels at nearly the speed of light in a vacuum. In water, however, the speed of light is about three-quarters of its speed in air, and in glass or diamond it is even lower still. Many materials will also absorb much of the infrared radiation.