Anyone who's tempted to go for conspiracy theories about deliberate delays, turn on CNN where they've been doing wall-to-wall coverage in excruciating detail from a counting centre in Pennsylvania and talking to officials there and in Georgia.
It's pretty much blow-by-blow: "And now we've been able to start on this pile of 3572 ballots which are the mail-in ones received on Wednesday and have already had the postmark checked to show posted before Tuesday, and the signature checked, but where the ballot paper got damaged by the letter opener and can't go in the machine and has to be transcribed before it can be counted. We have a block of 2652 ballots which arrived from Whosit County on Monday but of course the court didn't allow us to start verifying signatures to prepare for the count until polling day itself, and we're only just starting on those now."
It's absolutely fascinating and they're clearly working very hard.
In Georgia (I think), they were explaining that they've received back 8000-odd ballot papers fewer than they mailed out. They're expecting to receive a number more than zero but less than 8000 from those outstanding papers, and as long as the postmark is on or before 3 Nov (IIRC but do check if you care about the detail), the vote will be legal.
Each state has different laws about dates for accepting ballot papers and when counting can start, so it's hard to keep track!
The Georgia folk were working down spreadsheets reading out exact number of ballots outstanding by district, and giving a running commentary from local knowledge, "But that district has a large military base, so that figure is larger than you'd otherwise expect."