Both companies generally have a 97 percent delivery rate, ShipMatrix research analyst Mark D'Amico told ABC News. During the week of Cyber Monday, however, they both received a 91 percent rating by ShipMatrix.
"If you compare 91 to 97, that's a pretty big difference," D'Amico said, noting that he believes consumers "already have" felt the pinch.
In most cases, delayed ground service means that a package arrives one day late, D'Amico said.
Steve Gaut, the vice president of public relations for UPS, takes issue with ShipMatrix's figures, saying that he "will not validate the numbers presented by the external shipping consultants."
Gaut did note that "there were a couple of high impact areas" where e-commerce volume "came to us at levels higher than we expected for those specific locations," prompting UPS to bring in more staff.
"The vast majority of UPS customers are receiving their packages on time per the scheduled shipping commitments," Gaut said in the statement.
UPS reported that it found a 10 percent increase in the number of packages the company delivered between Black Friday of this year and the end of December 2014. And last year's figures were 10 percent higher than the prior year, according to UPS. All told, UPS expects to deliver 630 million packages during the holidays this year.
FedEx spokeswoman Sharon Young noted that changing online shopping patterns are impacting its business and causing "volume surges earlier than ever before."
"Our networks are performing as designed, and we are working to absorb the extra volumes," Young said.
Even though there was a delivery delay following Cyber Monday, D'Amico said there is no clear indication that it means that shoppers should have to worry.
"Just because the performance was down last week doesn't mean that it's going to remain throughout the year," D'Amico said.
"Cyber week is going to be one of, if not the busiest week of the year for shipping. That's not an indicator that it's going to stay that way through Christmas," he said.