“We’ve been doing network testing over the past few days to optimize the performance of video applications on our network,” a Verizon spokesman told Fortune. “The testing should be completed shortly. The customer video experience was not affected.”
Netflix (NFLX) said nothing on its end would cause the slowdowns. “We don’t cap data and don’t cap for any mobile network,” a spokesman said. “We offer settings inside the Netflix app to empower our members to control their own quality preferences and data usage.”
Fortune also reached out to Google (GOOGL) for comment and will update this story with any response.
The complaints, which arose in discussion forums on Reddit and other web sites, come at a critical time in the debate over net neutrality rules, which ban Internet service providers like Verizon from throttling access to specific web sites or online services. The ban is supposed to prevent ISPs from seeking extra fees, disfavoring competitors, or otherwise taking advantage of their powerful position standing between customers and the entire Internet. But the rules allow ISPs to conduct nondiscriminatory “reasonable network management.”
Get Data Sheet, Fortune’s technology newsletter.
The 2015 rules were passed under Federal Communications Commission chairman Tom Wheeler, an Obama appointee. New Trump-appointed chairman Ajit Pai, a former Verizon lawyer, is trying to roll back the rules.
Get Data Sheet, Fortune’s daily tech newsletter.
Earlier this week, some Verizon wireless customers took to Reddit to say that the speed of video streaming to their phones when they tried to watch shows on Netflix or YouTube appeared to be capped at 10 megabits per second, considerably slower than the speed their received for other apps or if they connected to the two sites in ways that masked the address from Verizon.
“This is a clear issue,” a user called crux57 posted on Reddit after measuring a speed difference of 50 Mbps. “No where does Verizon state they throttle to 10mbps. Not cool Verizon.”
Not everyone was accepting Verizon’s explanation. Net neutrality supporter Evan Greer, the campaign director at the Fight for the Future advocacy group, reiterated her call for maintaining the net neutrality rules. “Big Cable companies like Verizon have been caught repeatedly abusing their gatekeeper power in ways that hurt consumers,” Greer said. “This is exactly why we need net neutrality rules.” Conceding that not all details about the current situation may have come out yet, Greer added “the one thing that is clear is that these companies want more control over what we can see and do online.”
Some wireless carriers disclose that they downgrade the speed and quality of all streaming video as part in their standard “unlimited” data plans. T-Mobile (TMUS), for example, only allows DVD-quality video streaming unless customers upgrade to a premium version of the unlimited plan. But Verizon (VZ) has touted its unlimited plan as including full-quality video streaming.