Mail-in ballots are counted by scanners just like in-person votes, they aren’t counted by hand. Why would it take longer? Maybe he means that once all the votes are counted and they figure out how many they need to produce in order to make Biden the winner, that might take longer.
From another article:
Starting at 7 a.m. on Nov. 3, the ballot envelopes will be put through sorting machines that Philadelphia bought after inspecting similar equipment used by Orange County in California. The machines scan the outer envelopes and reject any that lack a signature. Each envelope is then sliced open on another machine.And that's in Philadelphia, where they've bought the fancy machines. More rural counties have a more labor intensive process.Suction cups separate the two sides of the envelope, and an election worker extracts the secrecy envelope. If it’s missing, the “naked” ballot must be tossed out, a court has ruled....
The secrecy envelopes are put back into the slicing machine to be cut open. Workers then remove and unfold the ballots, one by one, and insert them into scanners that tabulate the votes.
Now multiply times 3 million ballots.