First-class mail has been taking longer to reach its destination since the financially struggling U.S. Postal Service relaxed its delivery standards in an attempt to cut costs two years ago, according to a federal study.

Performance has generally declined for first-class mail this year, with the USPS missing its target delivery times at a greater rate than before, the Government Accountability Office said in a report this week.

The Postal Service revised its delivery standards in July 2012, increasing the number of days it gives itself to haul mail wherever it is going. For instance, if the distance was a 12-hour drive, the agency would previously try to deliver the item within two days. Now it shoots for three days.

Since the changes took effect, delivery times have grown for some categories of mail, the GAO said. Ninety-one percent of single-piece letters and post cards reached their destinations within an intended three- to five-day window during the third quarter of this year, compared to 95 percent during the same quarter in 2012.

The USPS has estimated that one quarter of its first-class mail volume was affected by the changes in delivery standards, according to the report. Periodicals and packages were unaffected, the agency said.

Aside from revising its delivery guidelines in 2012, the Postal Service has also shuttered many of its mail-processing centers in recent years. Since 2006, the agency has closed nearly 300 of the facilities, including 141 after 2011. It plans to shut down another 82 plants next year as part of a larger plan to reduce costs by $20 billion over the next few years — the next phase of closures is expected to save $750 million annually.

The USPS has also reduced its workforce through attrition from 788,000 to 491,000 since 2000.

Sen. Tom Carper (D-Del.), a leading advocate for postal reform and chairman of the Senate Homeland Security and Government Affairs Committee, said in a statement Monday that continuous cuts to services “affect the Postal Service’s ability to thrive and provide a dependable service that all Americans and businesses can count on.”

The agency downplayed the findings in the report, saying in a response letter that they do not represent performance across all postal districts. It also said that severe winter weather may have contributed to longer delivery times during the first half of 2014.

The Postal Service has been bleeding red ink in recent years, losing billions of dollars annually in large part because of declining mail volume and a 2006 law that requires the agency to prefund its retirement benefits at a cost of more than $5 billion a year — the USPS has defaulted on its last three annual payments.

Carper, who joined Sens. Heidi Heitkamp (D-N.D.) and Jon Tester (D-Mont.) in requesting the GAO report, said Congress must take swift action to help the agency regain its footing.

Despite widespread calls to overhaul the Postal Service in recent years, an agreement on legislation has eluded lawmakers. Several bills have been proposed, including a bipartisan measure from Carper and Sen. Tom Coburn (R-Okla.), but none has been approved by both chambers of Congress.