Cosmic microwave radiation is wideband and very low level and nearly completely isotropic, coming in from all directions. This is a booming loud signal by comparison, at a wavelength 100X longer than the peak of cosmic microwave and coming in from a single direction.
Cosmic microwave radiation slams as well as any radiation source. The “temperature” refers to source temperature of an equivalent black body source. There is a theoretical model of what black body radiation looks like, and laboratory sources match it quite well. The Universe also matches the model, if the current temperature of the Universe is 2.7 K. See: http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation
What I do not understand is this: 3K is by definition a very, very low energy state. Thus, how can we say that this low energy state “slams”?