Could it be that the "Ultimate weapon" seen at the end of ROTS was actually the Death star 2 under construction? .
It might be that the emperor also became impatient half way through with advances in weapon technology allowing the construction of the DS1 far faster with some cutbacks (such as a necessary emergency vent for radiation

However after the destruction of the ds1 then the weakness in the design would need to be removed in the ds2 (which would allready be built partially up to that point) as well as Palpatine wanting the superlaser to be retrofitted to allow greater targetting then the DS2 could be completed.
So, is this a decent explanation for the 20 years to finish ds1 (150 km), 3 years to completely build the ds2 (900km, better design, more weaponry, better power efficiency and an improved laser) brainbug? or is there some bit of eu or lucas level canon contradicting it?