Inspired by a recent conversation, and again a question that I get every now and again.
My suggestion is to weight your reading towards the easy (anything you can read with 98-100% comprehension, at a good reading pace), with some intermediate (anything you can read with 90% comprehension, and occasionally might need to pause to figure something out, or look up a very occasional word.
This should be your staple, for language acquisition purposes.
What about hard? Intensive reading? Figuring out that damnable Horace?
Here’s my question: Is there any pressing reason for you to read texts beyond your current proficiency?
If the answer is yes, e.g. you’re doing a course, you’ve got exams, you’re translating something for money, you’ve got a life-geas to understand Horace, then yes, spend some time in intensive/hard reading. Look up every word, diagram those sentences, get out your Loeb, do whatever it takes to make that text understandable. And then, read it, and re-read it, and make it your own. Tame that text. Memorise it. Domesticate it.
But if the answer is no, then why? From a language acquisition perspective, the time you spend toiling over figuring out 20 words of poetry, might have been spent reading pleasantly and rapidly through 20 pages (well, maybe not 20) of not so difficult material, which is still building up your language proficiency, still giving you input, still working you towards the day when those 20 lines will make a lot more sense, with a lot less effort.
So, if time and circumstances are on your side (and even if they are, to some extent, not), I prefer to weight readings towards the easy.