Creating APFS snapshots is basically instantaneous. Depending on how it’s implemented, restoring may also take no time at all or may take some time, my guess would be equivalent to how long the install took. Doing this for macOS updates is reassuring, having the option for any kind of install or change would be even more so. I think one can already make a snapshot whenever so it should only require the Recovery partition to give the option to restore any snapshot, not just the last one triggered by an OS update.
It’s a niche issue but I wonder if restoring will break a Mac’s binding to Active Directory, making logging in with an AD account fail. I’ve had this happen to Windows virtual machines after restoring their snapshots; domain controllers don’t want to see clients “go back in time.”