A quick process like that is pretty much insignificant compared to a month or two, let alone 15 years. Unless there are tens of thousands of other steps in the chain of comparable length, it doesn't come close to explaining it.

As I see it, there are roughly four steps:

1. Excavating.
2. Refining.
3. Power collecting.
4. Manufacturing.

The ones towards the end seem to be the biggest time sinks. However, power collection should not raise it by more than a factor of two or so. I don't think it takes many months to mine enough coal to pay for the energy costs of coal mining equipment, for example.