Light slows down any time it travels through a dielectric medium, such as water or glass.
Light appears to slow down near a significantly large gravity well. For example, an observer who watched her brother's spacecraft fly into a black hole would see her brother's actions gradually slow to a stop as he fell inside. Meanwhile, because the brother is in the bend looking out, he would see his sister's actions, along with those of the rest of the universe, speed up and race toward the distant future. Light slows down in a network, as anyone who has tried to download a file from the Internet on Sunday evening knows. If light, even at its finite velocity, could travel unimpeded in an electronic network, then you could still download a file on a server in Tokyo to a pc in New York in .04 sec.