It is slower only from a certain viewpoint. If we consider two tubes, one that is a vacuum and the other is filled with a gas. If you shoot light down both tubes you’ll find it traveled from one end of the tube, to the other, faster in the tube that is a vacuum. But from another perspective, light ONLY travels in a vacuum, so it must be the same speed. And the difference happened because in the tube full of gas, the light was being constantly absorbed by the gas, then remitted which will adds a small amount of time to the total journey, every time it interacts with a molecule in the gas.
Like two similar airplanes. Suppose one day we learn that they both left LA as the same time but arrived in NYC 2 hours apart. We might assume one is faster than the other; but after a little investigation we discover that the “slower” plane had a 2 hour layover in Denver and the “faster” plane just went to NYC without any stops. Someone might say that both planes did move at the same speed (while they were flying).