But that is fundamentally what agile is about. It's not about coding faster, it's the recognition that the specs are incomplete or wrong because fundamentally, a lot of customers cannot tell you what the want until they see it. That's why "build something simple and iterate on it" works. Regardless of how good your spec is, once the coding is done the customer is going to realise that that's not what they actually wanted.
But that is fundamentally what agile is about. It's not about coding faster, it's the recognition that the specs are incomplete or wrong because fundamentally, a lot of customers cannot tell you what the want until they see it. That's why "build something simple and iterate on it" works. Regardless of how good your spec is, once the coding is done the customer is going to realise that that's not what they actually wanted.