[Pontiac]

Speed of light in vacuum is not a function of wavelength.

You are correct. However not all Radiation is light.

In a Supernova a number of nuclear reactions occur that emit things like neutrons, protons and alpha particles. These types of radiation do not travel through a vacuum at the speed of light.

Also Supernova do not happen all at once. It is a process that happens over a period of time (days).

[NorthMountain]

In a Supernova a number of nuclear reactions occur that emit things like neutrons, protons and alpha particles.

Right. But the gamma will show up at the same time as the visible.