I don’t know the history, but I would guess that 18 minutes goes back to the time that everything was done in the same general work area so there was a very real possibility of the guys doing the aricha coming into contact with some dough from the lisha, hence the idea of pulling the plug every 18 minutes. Today, when there is an assembly line style process, that isn’t a concern, but the 18 minute standard was retained. You could probably go another 4 or 5 minutes without an issue by setting 18 minute timers for each stage of the assembly line. So 22 minutes, although not the usual standard, is probably not the end of the world. Whether the 18 minute shutdown is relevant or not, it’s certainly cant hurt to shorten the process in light of the Rosh, anyhow...
Actually after thinking about it I realized that the time from when the flour and water is poured until the first dough hits the table with the pole is pretty minimal- probably under a minute. So the only way to extend he time would be to do things in sequence- sending the dough to the 5 or 6 work tables with the poles batch by batch in sequencial order. The last batch would go to the last tables, which wouldn’t have come into contact with the dough until a few batches after the clock started. That’s probably overly complicated, and it makes sense that they choose to just shut everything down after 18 minutes, as opposed to machine matzos where if the process from start to finish takes 4 or 5 minutes, they can extend the 18 minutes by that amount without any problem...