Verizon and AT&T customers may now be able to sign up for unlimited data plans -- but a new report says their mobile data speeds are suffering because of it.
The "State of Mobile Networks" report, a crowd-sourced report of network providers released on Aug. 2 by network signal-reporting app OpenSignal, found that 4G data speeds for Verizon and AT&T have declined since the two companies reinstated unlimited data plans in February.
Those on the Verizon network were hit hardest. In the six months since the last OpenSignal report, LTE download speeds dropped 12 percent from an average of 16.9 megabits per second (Mbps) to 14.9 Mbps. AT&T dropped from 13.9 Mbps to 12.9 Mbps.
The reason for the decline is not necessarily due to network inferiority, but to network popularity.
Verizon and AT&T have twice the number of subscribers as their competitors T-Mobile and Sprint, reports Recode. The influx in network traffic after releasing the unlimited plans was significant enough to slow data speeds.
T-Mobile and Sprint, on the other hand, saw increases in mobile data speeds despite having allowed unlimited data plans for years.
According to the OpenSignal data, T-Mobile performed the best in all categories including 4G availability, download speed and data delay. Its LTE download speed came in at 17.5 Mbps.
T-Mobile's success is not without controversy, Recode reports. Some people have accused the company of limiting video quality to certain users to lighten web traffic, which they say contradicts net neutrality rules of treating all web traffic equally.
Verizon and AT&T also contest T-Mobile's top spot by doubting the validity of the OpenSignal data.
On July 25, testing firm Rootmetrics released a mobile speeds report that crowned Verizon as the top mobile provider, ranking it first in performance, speed, data, reliability and call quality in the U.S. Verizon, AT&T and Sprint all tied for texting quality, while T-Mobile's rankings remained unchanged from the previous report.
"Despite reports suggesting that unlimited data plans are stifling speeds due to increased demands on the networks, we are not seeing evidence of that in our first half 2017 results," Rootmetrics Director Annette Hamilton told Fortune, contradicting OpenSignal's claim made one week later.
All four major carriers issued statements favoring the company that ranked them higher.
"If you look across all of the network tests, including the drive test results from RootMetrics -- which is the most scientific available -- Verizon continues to have an overwhelming lead over the competition in network performance for all aspects of a customer’s experience," Verizon said in a statement. "Individual peak speeds from crowd-sourced tests, which OpenSignal does, don’t tell the whole story."
"We constantly monitor our network performance," a spokesperson for AT&T said. "The launch of our unlimited data plans has not impacted wireless speeds on our network."
John Legere, T-Mobile CEO, wrote a Twitter post expressing his enthusiasm for the OpenSignal data: "Who can handle #unlimited? Not the other guys. More reasons [T-Mobile] has the best unlimited network! #ReadEmAndSweep."
Sprint said it found "value in all of the various tests and they show similar results [that the network has improved]." It noted its network's performance was improving while AT&T and Verizon were slowing down, Fortune reports.