I should say, the speed of tabulation. An American election can include ballot lines for president, senator, member of congress, state senator, three state representatives, a county councilmember or two, a member of the board of education etc.
You forgot about 2000. Also, the main reason for the delays are mail-in ballots, which could be delayed for days/weeks, depending on how lenient the deadlines are.
By law the mail-in ballots have to be in by election day.