This question already has an answer here:
In the end of The Revenge Of The Sith, Darth Vader and Palpatine are watching the construction of the first Death Star. This scene probably takes place less than a year after the previous scenes (according to the answers to this question), but it would still be 18-20 years before it is fully operational in A New Hope. However, the second Death Star only takes about 5 years to build (assuming that they weren't expecting Death Star I to be destroyed)(the time between episodes IV and VI).
The first one was much smaller (2-3 times), but still took 5 times as much time to finish. Why?
Ok, the first one was a first, and had glitches, but surely it wouldn't take 5 times longer to build the first version then to improve the design massively, and increase the size!
Note: As I consider Disney to have seriously messed up canon, I would prefer Legends answers if the films (I-VI) do not supply one.