I read all the time, mostly fantasy and science fiction, and usually when a book feels like a slog is when I feel the author doesn't have the best prose, and prose is literally their use of words to tell a story.
Some authors are just gifted with the ability of storytelling, and those are the authors I tend to read a lot.
Lately, I've been getting through Brandon Sanderson's The Stormlight Archives, which is an excellent epic/high fantasy series. (Yes, epic/high fantasy is the same thing), and I've been reading John Gwynne's new trilogy The Bloodsworn Saga, and I'm on the second book of that series. Also great fantasy.