Can you direct us to contradictory documentation?
I have recently read such documentation but I don't remember where. I will see if I can dig it up, but don't hold your breath as it may take a while.
I just tried a few command-R restarts on my Mid 2015 15" Retina MacBook Pro/2.8 GHz/16 GB RAM/macOs 10.14.5, and they averaged out at about 55 seconds to desktop, whereas a few restarts into my boot volume averaged out at about 20 seconds to desktop.
On my system with a large conglomeration of external drives and volumes the normal time to boot to the desktop matches your twenty seconds, but it will take at least three minutes for everything to be up and running. The same is true in either Mojave or Catalina.
