you should remember the equation for electromagnetic waves :

v = fλ

where v is the speed, f is the frequency and λ is the wavelength.

The speed of light and infrared radiation doesn't change. (i.e v is constant)


Since infra-red radiation has a longer wavelength, the frequency is shorter.

Recall that infrared radiation (IR) extends from the nominal red edge of the visible spectrum at 700 nanometers (nm) to 1 millimeter (mm).

The former is known as near IR and the former is known as far IR


So in a sense its speed can be taken to be the same as light's by looking at the near IR portion.

https://www.britannica.com/science/electromagnetic-radiation

The speed of electromagnetic radiation of all kinds is the same universal constant that is defined to be exactly c = 299,792,458 metres per second (186,282 miles per second).


Of course, this is taken to be in a vacuum (which space is)