From the WSJ, which is closely tracking this issue:

That puts critical votes at risk in an election that, more than any before, will hang on mail-in ballots. To find choke points for those ballots, the Journal is tracking millions of letters traveling across the U.S. in near real time….

Of the 29 states that require mail-in ballots to arrive on or before election day, 28 have since late July seen periods of average delivery times exceeding six days, the Journal analysis found. Among them are battleground states like Florida, Arizona, Wisconsin and Georgia….

“It’s really hectic right now, crazy busy—there’s just a lot of ballots,” said Michael Moriconi, president of the Tucson, Ariz. branch of the American Postal Workers Union. “We don’t have enough people.” The APWU’s national executive board endorsed Joe Biden in June.