Well it is really not true. The speed of light is a constant in a vacuum. Its speed is different in other media. It can go slower in crystalline material. In fact the slower it goes the greater the brilliance of a cut stone.
Some lead glasses slow it down even more.
This is just like sound, going different speeds through different media. Imagine that you have a charged comb and you wave it up and down. Then you are creating electromagnetic radiation. If you move it up and down once per sec then you are creating a wave with wavelength equal to 300,000 km. This is a very low energy wave.