I can't think of a book that's now considered as a classic to be not a slog. All of them were written in a different style from what we are used with today's writing style. Style, prose, language changed over time. Not sure if literature followed people's tastes or vice versa.

I find her books quite slow and difficult to follow.

I would have to say my grammar has improved without me knowing how or why, since the last English course that I took was in my college days. As you write, you just know that a sentence is structured correctly without knowing the grammar rules. Or maybe you know the rules. Just on a subconscious level. Not really sure.