That actually sounds like most of the issues you are having are related to Rachio not having pulled in the iOS10 compatible blinkup library - which isn’t necessarily surprising as gen1 isn’t their current product. The symptoms (irregular length blinking) fit this. @franz, something worth doing when the app is next updated maybe?
A guest network won’t slow down wifi in any measurable way; essentially this just makes the AP transmit two beacon packets (one for the normal network, one for the guest network) every 100ms - that doesn’t account for much airtime.
I agree with not touching it if it’s working now