– Amid a significant downsizing of the money-strapped U.S. Postal Service, the number of letters arriving late has jumped by nearly 50 percent since the start of the year.

And that's as measured against the agency's own newly relaxed standards.

The delays have become so serious that the Postal Service's watchdog issued an urgent alert earlier this month recommending that postal officials put all further closures of mail-sorting plants on hold until service stabilizes.

"The impacts on customer service and employees have been considerable," Inspector General Dave Williams wrote.

Mail that's supposed to take two days to arrive took longer — anywhere from 6 to 15 percent of the time during the first six months of 2015, investigators found, a decline in service of nearly 7 percent from the same period last year. Letters that should take three to five days took longer anywhere from 18 to 44 percent of the time, a 38 percent decline in performance over the same time last year.

First-class mail has gradually been traveling more slowly since the Postal Service started closing dozens of mail-sorting plants in 2012. But in January, something more drastic happened: To prepare for another round of plant closings, the agency eliminated overnight delivery for local first-class letters that used to arrive the next day. And up to half of mail traveling longer distances was given an extra day to reach its destination.

These longer delivery times became the new normal, or "service standards" in postal parlance. Mail was considered on time if it took four to five days to arrive instead of three.

But postal officials have struggled this year to meet even these lower standards. The delays have been compounded by two factors, the inspector general found: Severe storms last winter and changes to plant operations that started when the new standards took effect. Thousands of postal workers were reassigned and shifts were changed, resulting in a disorganized, inefficient workplace.

From January through June, 494 million pieces of mail did not meet the standard for local or cross-country delivery, a 48 percent jump from the same period last year.

A dying business

Snail mail still is a dying business for the post office, which is ramping up its e-commerce business with packages and same-day delivery of everything from groceries to Amazon orders. But the mail still matters to Americans, who sent or received 155 billion pieces in fiscal 2014. Checks, medicine, magazines, mail ballots, newspapers, greeting cards, court documents — they're all in the mail stream.

"The volumes are still immense," said Steve Hutkins, whose blog Save the Post Office reports on consolidations facing the postal system. "There's a lot of important stuff in the mail."

The slowdown "is a huge issue for many nonprofits that rely heavily on mail to fund their critical missions," said Stephen Kearney, executive director of the Alliance of Nonprofit Mailers, calling mail a "lifeline for curing diseases, helping returning veterans, informing consumers" and other services.

Service gradually has rebounded a little each month since January, with scores for two- and three-day mail within .914 and 10.9 percent in June of where they were during the same month last year. But the inspector general cautioned that mail still is not reliable.

Postal Service spokesman David Partenheimer, in a statement, described the changes in January as the "greatest operational changes the Postal Service has ever implemented.

"Despite our best efforts to minimize the impacts of the changes, there were some insurmountable challenges that negatively affected service performance, especially when considering the impacts of severe winter weather conditions," he wrote.

"We remain totally committed to identifying and correcting errant processes in our operations as early as possible."

But the agency did not agree with the inspector general's recommendation that plant closings stay on hold until service improves across the board. Postmaster General Megan Brennan has temporarily halted the closings; it's unclear when they'll resume.

In recent years, the Postal Service has tried to shutter thousands of post offices and end Saturday delivery to save billions of dollars. Politicians stopped these efforts because they weren't popular with constituents. The consolidation of mail-sorting plants went further before anyone really started to feel the effects.