Definition
Throttling is the deliberate slowing of network traffic by a network operator. ISPs throttle for various reasons: to manage congestion at peak hours, to enforce data caps after a threshold, to prioritize their own services over competitors', or simply because their commercial agreements with customers permit it. Common targets are video streaming, peer-to-peer file sharing, online gaming, and VPN traffic itself.
How a VPN Defeats Protocol-Based Throttling
If your ISP throttles BitTorrent, they identify it by its protocol signature in your unencrypted traffic. A VPN encrypts everything inside an opaque tunnel — the ISP can no longer tell whether you are torrenting, streaming, or browsing, and cannot apply protocol-specific throttling. Many users discover that BitTorrent runs faster through a VPN than directly because their ISP was secretly throttling it.
When VPNs Cannot Help
If the ISP throttles all traffic equally past a data cap, a VPN cannot recover speed. If the ISP throttles all VPN traffic equally (as some do in restrictive markets), a VPN with obfuscation can sometimes restore speed by hiding the fact that it is a VPN. If the underlying connection is genuinely congested, the VPN cannot manufacture bandwidth that does not exist. Throttling-based slowdowns are the case where VPNs help most; capacity-based slowdowns are the case where they cannot.