At least, until I started learning Japanese. After a few years of that, I realized that the pain I was feeling while reading Japanese kids' books is what all kids feel when learning to read initially. It's just been so long ago that I've forgotten it.
In short, reading slowly is very painful. You know that others can read quickly, but you can't. You know that there's a good story there, but it's constantly interrupted by trying to pronounce things, trying to remember what words mean (or worse, looking them up!), or skipping them and trying to figure out what they mean from context.
It's way more painful than you remember.
Something that encouraged me to read with outside motivations would be welcome to me. So that Kindle kids app actually sounds awesome to me as a middle-aged adult right now.