That includes the lag time. A single dose is supposed to be about 80% effective once immunity develops. They don't know how long that will last and thus have the second shot to increase effectiveness and hopefully longevity.
It's likely that the 80% figure you mention was based on protection against "ancestral type" virus, as would have been measured in the main trials. The gap between first dose and second dose protection is larger for Delta variant than for other variants, as documented in the paper I cited.