I suspect a lot of it has to do with the kinds of books one reads.

Many years ago a friend claimed to read a book a day. She was referring to cheap paperback fantasy books, her preferred genre. She could read them very fast because she already knew what was in them. She told me she mostly read the dialogue, which would definitely cut down on the time spent reading.

So perhaps the OP is reading mostly books they're already familiar with -- novels in a genre they like and nonfiction on topics they already mostly know. If so, you might be able to do similar things.