Perhaps I should define terms. In SLA research, "extensive reading" specifically means reading material where you know at least 98% of the words in the text. At that level you can generally follow the text without a dictionary - including figuring out the meaning of new words and expressions from context - so almost by definition it's not a slog.
It's not just a second language acquisition thing, either. At least in the USA, many educators recommend younger readers do the same thing when reading in their native language, in part to save them having to repeatedly refer to a dictionary.
It's not just a second language acquisition thing, either. At least in the USA, many educators recommend younger readers do the same thing when reading in their native language, in part to save them having to repeatedly refer to a dictionary.