Some people are just really gifted readers. Whether through some combination of learning to read early or just being able to skip the ‘sub-vocalization’ part of reading, many are able to get through hundreds of books a year. It’s a kind of super-power if you think about it.
I don’t think I’ve gotten faster at reading as I’ve aged, but I’ve certainly gotten better at skimming through texts. If you’re relatively familiar with a subject you’ll know which sections of text you can skim over and which sections are worthy of careful reading