Uber convincing argument is "light can't escape a black hole" .. ???As was said earlier light takes longer to pass through water even air but that isn't because it is slowed down it is because it gets bounced around a bit so it takes a slightly longer route rather than a direct one.
However light is affected and is slowed down by gravity as is time, for instance it takes a photon of light in the form of a gamma ray somewhere between 10,000 — 170,000 years to travel from the core of the sun to its surface, a journey that should only take 2.3 seconds.