I mean, pretty much any complicated engineering project should be quicker the second time around, because the technical challenges have already been solved and the infrastructure to build more is already in place.
A “not quite analogous but analogous enough” example is the Atomic Bomb. The first atomic bomb took about 5 years to go from concept to first successful test (and arguably 20 years before that to lay the theoretical groundwork). But the hydrogen bomb, a much more powerful weapon, only took about 2 years to develop (started in 1950, first detonation test in 1952). This is because most of the engineering and theoretical problems were already solved, and most of the infrastructure was already in place to build more bombs, etc.
This doesn’t necessarily apply if you’re building a much more advanced/improved version of something with fundamentally new capabilities, instead of just duplicating or scaling up something. Like the F-22 fighter jet took around 6 years to develop, but the much more advanced F-35 took over 20 years. That’s because the latter was such a leap forward and so ambitious that it introduced many new engineering problems, and ended up getting bogged down in defects and budgeting issues.
The DS2 should be quicker to build than the DS1 unless it’s doing something way more advanced than the DS1. We know the DS2 is bigger, it can fire the superlaser at an angle, and I think it recharges faster. So there might be some new engineering challenges. There’s no way to say for sure since it’s all fictional. But the MAIN challenge (scaling up a laser to such ridiculously insane energy levels that it can blow up an entire Earth-sized planet) is already solved.
Plus, with the DS2 you also have other factors that should make it quicker to build than the DS1, such as less political obstacles due to the Senate not existing, and more urgency to get it done. (ROTJ begins with Vader showing up and threatening an Admiral to speed up construction.) I bet the DS2 would have had tons of defects if it was actually completed, due to being so rushed by Palpatine.