https://www.post-gazette.com/news/politics-local/2022/08/17/butler-county-ballots-review-2020-election-vote-recount-elections-staff-scanners-pennsylvania/stories/2022081701251,661 ballots from the 2020 election were recounted both ways.
Machines: no counting errors; 1 undervote that was human adjudicated.
Humans: 9 people took 18.5 hours (about 170 hours of labor), 2 counting errors.
That's for 1,661 ballots. The review process they used (see link for details) was slower than an actual election-night hand count would be...but not a lot slower. In the 2020 election, Pennsylvania had almost 7 million ballots cast. The U.S. had over 158 million. You do the math.