In December, amid a crush of packages and record numbers of coronavirus cases, service performance across the U.S. Postal Service network plummeted to the lowest levels in years, with only about 64 percent of first-class mail delivered on time around Christmas.

The Postal Service was under scrutiny for months leading up to the election, when many feared that delays from operational changes would hamper the counting of mail-in ballots. By many measures, the Postal Service ultimately succeeded in delivering ballots quickly. But over the weeks that followed, during the peak holiday package season, the network grew more and more backlogged.

“Under some of the most difficult circumstances we’ve faced in the past century, the U.S. Postal Service successfully processed and delivered both a record number of mail-in ballots and a record number of holiday packages for the American people, amidst a global pandemic,” said Kim Frum, a spokeswoman for the Postal Service, in a statement that acknowledged the slow mail delivery in December.

The Postal Service delivered more than 135 million ballots and more than 1.1 billion holiday packages. But those extraordinary challenges have broadly strained the network.

“This is the worst I’ve ever seen it in at least the last 10 to 20 years that I can recall,” said Angelo Anagnostopoulos, the vice president for postal affairs at GrayHair Software, which tracks mail for large companies that send, prepare and transport it. Several other industry experts echoed that assessment.

December’s slowdown was the second significant wave of postal performance decline in 2020. Last summer, after the arrival of a new Trump-aligned postmaster general, Louis DeJoy, mail also slowed around the country. Citing the need for improved efficiency, Mr. DeJoy imposed policy changes that postal unions and many outside experts said slowed mail processing. After the resulting outcry, which prompted congressional investigations and lawsuits, the Postal Service reversed some of those policies, and delivery speed partly rebounded.

The Postal Service is a particularly opaque agency, normally reporting its performance only four times a year. But court orders preceding the election have forced it to publish unusually detailed data during a tumultuous time for the agency.

Those weekly numbers, down to the district level, show the parts of the country that particularly struggled in December. Worker shortages in sorting plants in the Lehigh Valley of Pennsylvania were so prevalent that only a quarter of first-class mail was processed on time. Other types of mail, like periodicals and marketing mail, had a one in 10 chance of arriving on time in some parts of the country.

These severe declines in service reflect what large-scale mailers say they have seen among their own shipments and clients — and what anyone who mailed holiday cards might have experienced as well. Letters that usually traverse the country in days have sometimes taken weeks. Some mail sent in early January was delivered sooner than mail sent in mid-December, when letters piled up in processing plants behind newly arriving packages and mail.

At clogged postal facilities, truck drivers dropping shipments for commercial mailers sometimes had to wait 12 hours or more to unload, and in some cases had to leave and try again the next day, said Kathleen Siviter, an official with the National Association of Presort Mailers. She described drivers refusing to transport mail to postal facilities, and companies that spent hundreds of thousands of dollars in extra transportation costs.

Delays like these had further consequences, in bill payments that arrived past due, in coupons that missed the sale window, and in nonprofit fund-raising pleas that weren’t opened before the end of the tax year.

Some of the factors leading to the December slowdown were unusual and are unlikely to persist: a huge, national wave of Covid-19 infections, which affected postal workers; increased parcel demand from the pandemic; the holiday rush; and difficulties hiring temporary workers at a time when competing shipping services were also hiring.

“December was the perfect storm,” said Paul Hogrogian, the president of the National Postal Mail Handlers Union. “Everybody was fighting over hiring, and we couldn’t hire.”

The Postal Service wasn’t alone in running behind schedule. Both FedEx and UPS also experienced some delivery delays in December, according to data from ShipMatrix, a firm that tracks parcel delivery for all three vendors.

All of this also followed an election when the Postal Service went to great lengths to prioritize millions of mail ballots. And in December, Michael Plunkett, president of the Association for Postal Commerce, says he believes the Postal Service then prioritized packages over mail. Packages take up more space in crowded facilities. But he cited another incentive, too.

“Those package mailers have recourse — they can go to FedEx or UPS,” said Mr. Plunkett, who is also a longtime former postal employee. “If you’re a mailer, it’s a monopoly, and you can’t go anywhere else.”

If mail performance doesn’t improve quickly, many in the industry worry it could have long-term consequences for the way the Postal Service is viewed and how much it is used by commercial mailers.

“I want people to have confidence in it, and I think they’re losing it,” said Dave Lewis, the president of SnailWorks, another company that helps commercial mailers track their mail. “People are saying, ‘No, don’t send it by mail, send it some other way.’ That’s very destructive to us in the long run.”

SnailWorks began sharing data from some of its customers with The Upshot last summer, to help measure the performance of the Postal Service ahead of the election. That data, which tracks individual pieces of mail by bar code, shows not just a December surge in late mail, but also a spike in mail arriving five or more days late.

The SnailWorks data, which isn’t representative of the full postal network, is more current than the official Postal Service data. And it doesn’t yet show much improvement.

“I really thought by mid-January we would have started to see real signs of progress,” Mr. Plunkett said.