Addition is fast. Opening ballots, verifying signatures, checking voter is legit, and then marking it all down takes time. Especially when bound by law to not open envelopes until 7pm eday.
Let's say it takes a minute to process a single ballot. That means a single poll worker can go through a couple hundred ballots--500 is a nice round number-- a day. And all of this is going to be processed generally at a centralized facility at county level, and because of the pandemic, you might have 500,000 of those to get through. Even with 100 workers working those ballots, that's still going to take 10 days to get through everything.