Martin had been working on ASOIAF for almost 10 years before it was released. And Game of Thrones was delayed to allow him to finish Clash of Kings and Storm of Swords. The gap between Storm and Crows was actually pretty typical for his pace, he's not a fast writer to begin with.
And with the higher expectations for Winds of Winter and Dream of Spring I think he's all the more a perfectionist to make sure everything is in place - I expect there is a high level of anxiety mixed in as well, which can be crippling for a writer.
Four years ago I talked with his editor, Ann Groell, at a Con, and she explicitly said she had read the ending of Dream of Spring. I think there are a lot of pieces that need to be put together, and I wouldn't be surprised if most of Spring is written, he just has no idea how to get there. He writes piecemeal, and she edits as he turns in, so it will just be a matter of putting everything together coherently.
That said, I'm hopeful for Winds late this year (and I expect Dream of Spring will be quicker to follow). The publisher will be able to throw the switch and have it on bookshelves in about 6 weeks because so much is ready and waiting (and they'll push it ahead of everything else to speed it up).
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