D.K.
I had a niece who was the VERY same way at about the same age. She could "read" a page from a book. We would read one page, she'd "read" the next. We thought she had just memorized it. She'd spell words with magnetic letters on the fridge.
We found out that her cognitive development was more developed than her verbal development. I guess in most cases, they develop pretty much at the same time. THIS is what caused the stuttering. She couldn't get out (verbal not developed0 what she knew in her mind. The doc said she'd outgrow it when her verbal development caught up. It did!