Here's my understanding about why modern dishwashers have longer cycles: (my friend's 1 year old dishwasher's shortest cycle takes 4 hours!)
The energy savings come from the fact that the dishwasher isn't drawing power the whole time it's running. Instead, it pulses on and off. The water savings come from the fact that in old dishwashers, water was being pumped simultaneously through both upper and lower sprayer arms. In the new dishwashers, there's less water, so it's only pumped through one arm at a time. This is also the reason why the cycles take so much longer.
https://youtu.be/_rBO8neWw04
My son sent me this video explaining everything you need to know about dishwashers. I found it totally fascinating and very educational. It also explains why those detergent pods are not the best idea. (He doesn't mention that the outer layer has a tendency to not dissolve completely and cause blockages in the pipes, but that's been the experience of several of my friends who have used them in the past.)