Verizon Wireless customers this week noticed that Netflix’s speed test tool appears to be capped at 10Mbps, raising fears that the carrier is throttling video streaming on its mobile network.

When contacted by Ars this morning, Verizon acknowledged using a new video optimization system but said it is part of a temporary test and that it did not affect the actual quality of video. The video optimization appears to apply both to unlimited and limited mobile plans.

But some YouTube users are reporting degraded video, saying that using a VPN service can bypass the Verizon throttling. The Federal Communications Commission generally allows mobile carriers to limit video quality as long as the limitations are imposed equally across different video services despite net neutrality rules that outlaw throttling. The net neutrality rules have exceptions for network management.