You have a super computer in your pocket. There is no reason counting votes should take any longer than it takes to receive them.
But, even if you ignore that two individuals (or three) representing all the candidates each count by hand a stack of ten, have to agree, sign off that they agree, etc. I think you underestimate other problems. You're focused on the counting.
Just consider how long it takes to open the outer envelope, check the signature on the inner envelope (again, by multiple people who represent different campaigns), and physically remove and unfold that sheet of paper. Now multiply by several million. Add in some extra time for managing stack of ten ballots.
Or, to put it another way, we don't vote with a phone app. We are willing to pay extra for security.
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.
I really suspect you’re being intentionally difficult here, this is not such a simple problem as you suggest.