This is a comment section quote from Tim Shanahan from his website. The article was on guided reading versus common core. "At kindergarten and grade 1, my advice is that you should not ramp up text difficulty on the reading end. I am a big believer in reading complex texts to kids (books that they definitely cannot read themselves), but with regard to beginning reading you want a mix of texts that expose kids to a high concentration of very high frequency words and that have a large percentage of words that can be decoded with relatively simple phonics (such as one-to-one correspondences of letters to sounds, and preferably non-conditional matches of letters to sounds). By the time students can handle high first grade level texts, then you can start to move them up in difficulty. Initially, keep your emphasis on mastering the decoding system. If you ramp up the text difficulty too early, I fear that you will slow that process down."
Text complexity is a BIG issue with common core and I think this clarifies his stand on how it effects students at the "learning to read" stage.
His website has some great information. www.shanahanonliteracy.com/