They have 20 auditing stations.
If they man all the stations, 24 hours a day, 7 days a week, each station needs to audit one ballot every 30 seconds or so.
They're not moving nearly that fast. (And they're not always using all the stations.)
Will they speed up as they get better at it?
They also have to find what the cheating ballots look kike. Once they know they can fly. They have probably only seen a few so far.