That's correct, sort of. Individual photons travel always at c (they don't have mass, so they literally can't go slower), but when traveling through a transparent (for their frequency) medium they don't go through the atoms or around them—they would hit an atom (its electrons, really), get absorbed, and then re-emitted. The average speed of all the photons will be lower than c, and depends on the frequency, that's how we got rainbows. For most frequencies, including visible light, it goes slower the higher the frequency is, but gamma rays/x rays photons are so high energy that they don't interact with the electrons, but with the nuclei—that's why you want materials like lead or tungsten to block them—which are much smaller, resulting in fewer absorbing-emitting interactions and a higher average speed.