We've talked about the Death Star thing in the Official thread already. There are a number of explanations for why it took 20 years. First off, it could be the prototype Death Star that was built at the Maw installation. It was basically a frame with a working main gun. That prototype was in one of the books, and the books are not canon, but it's an explanation.
Secondly, exactly how long does it take to buld a "small moon"? Just building the frame could take years, after which you have to fill in miles and miles of hallways, rooms, etc. It would be a huge task to say the least.
As for the second Death Star, we have no indication how long after the start of construction on the first one, they began to build the second. It may have been only a few years later, and it was just nearing completion at the end of Jedi. The second Death Star was said to be larger and more powerful than the first, so clearly they learned things on the first one that were than applied to the second. Building it more quickly may have been one of the things they learned.
