I’m carving this off into its own thread as I didn’t want it lost in the longer G3 thread. 
@emil , I understand it would be a software feature. As we’ve seen there may be other V3-only SW changes coming so it was worth asking.
My Hunter Pro-C was setup for 6 minutes of delay between zones and seemed to do ok. We had recently moved into the home before I swapped the Hunter Pro-C out for a G2 Rachio so I don’t have a lot of historical evidence to go off of. Our well pump is set to turn on when the pressure tank drops to 30 PSI and turn off when the pressure tank gets back to 50 PSI. The safety pressure relief valve opens at 75 PSI. Maybe when we turn our system back on this spring I’ll run our largest zone and see how long it takes to draw down to 30 PSI and how long it takes the pump to build back up again.
However, having a pre-specified delay in between every zone every time is how a dumb irrigation controller handles this. With Rachio being a smart controller I’d expect something a bit more elegant so you can put another feather in your fedora. If you chose to throw in the always delay between every zone feature to hush us up, ok that’s better than nothing and makes it so you can check a feature box. I’d understand. 
I’d like to see a smart controller able to do is give us the ability to enter some kind of “Maximum run-time before recharge” value as some some kind of “time to recharge” value. For example let’s say we knew we can run our largest zone for 15 minutes from our starting pressure (50 PSI for us) before hitting our floor value (30 PSI for us) and then it takes 5 minutes to recharge back to 50 PSI. We could enter 15 minutes & 5 minutes for the Rachio setup. Then, Rachio may be able to get multiple zones completed before pausing for a recharge if each zone was scheduled to run for 7 minutes that day. It would seem a bit silly to pause for 5 minutes between every zone in that situation. Maybe the system only stops to recharge if it knows it cannot finish the next scheduled zone without at least one recharge. Zones scheduled to water for >15 minutes would obviously need at least one recharge in the middle. You’d reduce the overall schedule duration if you allow the maximum run-time to cross the zone boundaries instead of always forcing a constant set value of delay between zones. You may also find the delay helping complete soak/cycle times if those times are at least equal to or longer than the recharge time.
There may however a slight positive trade-off by always pausing between zones in that you’d start each zone closer to 50 PSI rather than starting closer to the floor value then having to pause, recharge, and finish the zone. Another thought might be the user bases their max run-time value on how long it takes their largest zone to draw down to a PSI where their irrigation heads are no longer able to run at their own optimal PSI. This value may be higher than when their well pump would traditionally kick in.
In the ‘if we dream big’ category if a flow meter is installed you may now have a simplistic form of pressure detection. Admittedly details are scarce on the native Rachio flow meter. I’m making assumptions about its ability to transmit live data and how many data points it is able to capture in a period of time. One could theorize a “learn mode” that goes above and beyond the method I shared above. In the learn mode you would enter values for system’s starting PSI, the floor PSI (or the ‘do not go below’ PSI if you’re basing it on irrigation head performnace), and the average time (in mins) to recharge from the floor PSI value you set. You then start the learn mode test within the Rachio app after confirming no other system is pulling water in the home and your system is at the starting PSI. Rachio learns how many ticks per second on the flow meter are full charge, then it continues running the system while the human watches their pressure gauge until the floor value PSI is hit. When the floor value PSI is hit the user taps the screen of their mobile device in the app. This tap allows Rachio to learn how many ticks per second the floor value PSI is. The system now has the ability to extrapolate the PSI on the zone based on the ticks per second it is seeing on the flow meter as well as how long it takes to get from the start value to the floor value. You may have to run this learn mode on each zone due to differences in zone config as maybe different zones can withstand a different floor PSI due to # of heads and head type.
With this information Rachio could make intelligent choices on when to pause for recharge. It would also know when it should be safe to restart assuming there is no other system in the home actively drawing on the system at the same time. For our home the irrigation schedules typically run when we’re asleep so there shouldn’t be a dishwasher, shower, or any other contributing draw on the system while operating. Understandably that may not be the same for all households, but Rachio would then be smart enough to know it should stop what it is doing for a while and try again in a few minutes.
I may certainly be overlooking things that make all of this a silly idea, but that is kind of how I envision a smart irrigation controller going about this. You’d have some basic “dumb” methods and then some more advanced options for users who wish to go the extra mile or have systems with sensors that enable the added capability.