Your VPN speed test shows 20 Mbps. Your raw internet speed is 200 Mbps. That 90% reduction is not from encryption overhead -- it is from your VPN's congestion control algorithm choking on China's international links. BBR fixes this.
The CUBIC Problem
Most VPN servers use CUBIC, the default TCP congestion control algorithm on Linux. CUBIC works well on low-latency, low-loss networks. It does not work well on China's international links, which experience 5-10% packet loss during peak hours and 40-80ms latency to Singapore.
CUBIC interprets packet loss as network congestion. When it detects loss, it aggressively reduces the sending rate. On a network with persistent background packet loss (like China's international links), CUBIC spends most of its time in a reduced state, delivering a fraction of the available bandwidth.
How BBR Works
BBR (Bottleneck Bandwidth and Round-trip propagation time) was developed by Google in 2016 and deployed across Google's global network. Instead of reacting to packet loss, BBR builds a model of the actual network path:
- Bandwidth estimation: BBR probes for the maximum available bandwidth by periodically sending at higher rates and measuring delivery
- RTT estimation: BBR tracks the minimum round-trip time to estimate the network path's propagation delay
- Loss tolerance: BBR does not reduce sending rate in response to packet loss unless the loss indicates actual congestion (queue overflow), not random loss
On China's international links, where 5-10% of packets are lost due to link quality rather than congestion, BBR maintains 2-3x higher throughput than CUBIC.
Real-World Impact
In testing from China Telecom residential broadband to a Singapore VPN server:
- CUBIC: 15-25 Mbps during peak hours, 40-60 Mbps off-peak
- BBR: 60-80 Mbps during peak hours, 80-100 Mbps off-peak
The difference is most noticeable during peak hours (7-10 PM) when packet loss increases. BBR maintains usable throughput while CUBIC drops to near-unusable levels.
Does Your VPN Use BBR?
Most commercial VPNs do not advertise their congestion control algorithm. The default on most Linux servers is CUBIC. BBR requires explicit configuration on the server. If your VPN does not specifically mention BBR, it is almost certainly using CUBIC. KookVPN has BBR enabled by default on all servers.
The speed difference is real: BBR is not a marginal improvement. It is the difference between 480p YouTube and 4K YouTube. If you have been blaming your ISP for slow VPN speeds, the congestion control algorithm is more likely the culprit. Check our speed test results for real benchmark data.