For reference, the average residential route gets somewhere between 0-3 certified letters per day. The generally estimated time added each day per cert is ~2 minutes. At 500ish registered voters per route, this would have added about 16 hours 40 mins per route, or 24 hours 40 mins per day.
According to several people knowledgeable about the discussions, the administration initially floated requiring all mail ballots nationally to be delivered exclusively through certified mail (which would have required voters to be home and sign for delivery of their ballots).