There's no logical reason why it should take significantly longer to count the votes in a big election than in a small one. There are a lot more people voting, true enough, but there are a lot more people counting the votes. Adding up the separate counts could be a matter of minutes.

As for absentee votes there is no reason that should delay things more than a few days. If the votes come in later than the deadline, tough luck, they've missed the bus.

There's also the fact that US elections involve a lot more things than just the President, and they have these massive voting forms to confuse people but there's no reason to count all these at the same time. The vote for the dogcatcher and the rest of the list can wait.