Death Star delays
I recently read an article by Dave Filoni, saying that by the time of Rebels, ( 5 years before Yavin), the Empire was still building the Death Star. That being said, it was complete in 0 BBY, and was already in construction around 19 BBY, as at the end of ROTS, Vader and Palpatine were observing its skeleton. The Death Star likely took 15-19 years to build. Yet 3 years later, the second death star was halfway done. How did they increase construction time that much?
On an amusing side-note, I find it funny how each Death Star was destroyed earlier in it's construction than the previous one. Death Star I was complete and operational, Death Star II was incomplete but operational, and Death Star III wasn't even near being operational.
As far as we know there was a Death Star IV that had its plans destroyed before it had even begun to be built, and a Death Star V that only ever reached the conceptual stage before the person conceptualizing it was killed.