The wireless carrier had announced the policy in February ahead of the launch of the iPhone 4 on its network, but Droid-Life reports that the company has notified customers that, beginning Thursday, the practice has gone into effect.
"Beginning 9/15, we will start identifying customers who meet these criteria and will expand to others in the base after customer communications are completed. Network Optimization Practices only goes into effect when an Internet or Smartphone device with an unlimited plan/feature falls into the top 5% of data usage and is on a congested cell site," the company wrote in an email to customers.
Users marked for the periodic speed reduction will have the limitation last for the remainder of the current billing cycle and into the following. According to Verizon, the top 5 percent of data users on its network use 2GB or more of data each month.
A FAQ on the company's website indicates that full implementation of the program may take several weeks.
Verizon representatives are reportedly being told to recommend that customers either upgrade to a 4G Long-Term Evolution device, migrate to a usage-based (tiered) data plan or use Wi-Fi more often in order to avoid throttling.
In response to criticism over the policy, Verizon alleges that it is not throttling the same way as its competitors AT&T and T-Mobile are, as it is only slowing connections when users are on a congested cell site. In July, AT&T announced that it would begin reducing the speeds of the top 5 percent of its heaviest data users.
The new policy comes into effect within weeks of the expected launch of Apple's next-generation iPhone. Verizon has said it expects to participate in a simultaneous launch of the so-called iPhone 5 alongside AT&T, and possibly Sprint. The iPhone 5 is widely believed to sport a faster A5 processor and an 8-megapixel camera.