As far as I know all light speed measurements done using light sources that utilized electrons. The nominal light speed measurement used laser which is actually exited electrons. The speed of X-ray radiation from synchrotron used accelerated electrons. The speed of gamma radiation also involved with inhalation of electron and positron. The speed of RF is also involved with electrons modulation in coils and capacitors. Are there any speed measurements of electromagnetic radiation coming say from oscillated or accelerated protons?