There are two parts to absentee voting: validation and tabulation. Early in-person votes are validated when a voter shows up to the polls — usually a county courthouse or some similar location. However, mail ballots require officials to open envelopes and match the signatures on each voter’s affidavit with that voter’s signature of record. That’s a labor-intensive process that takes a long time.
Fortunately, most states begin the process of validating mail ballots early, meaning that by the time Election Day rolls around, automated tabulation is all that remains. Tabulation takes very little time. Severe delays are therefore likely only in states that prevent officials from validating votes until, on or very close to Election Day.
Several key swing states have a combination of experience with high rates of mail voting, and systems in place to begin validation and even tabulation well ahead of time. Florida, where large swathes of voters have been voting by mail for years, begins validating and tabulating absentee ballots more than three weeks before Election Day. Arizona, Colorado, and North Carolina do likewise two weeks early.
Georgia, Minnesota, and Ohio tabulate after the polls close, but their votes will be prepared and ready for a machine count.