If you've tested your connection with both IPv4 and IPv6 and noticed that IPv6 performs worse, you're not alone. Despite IPv6 being the modern successor to IPv4, many users experience slower speeds or higher latency when using IPv6. This article explores the technical reasons behind this phenomenon and provides practical solutions for diagnosing and fixing IPv6 performance issues.
IPv6 is not inherently slower than IPv4. The protocol itself offers similar or even better performance characteristics. However, in the real world, IPv6 often performs worse due to immature infrastructure, misconfiguration, broken connectivity, and less-optimized network paths. According to APNIC research, IPv6 connections exhibit an average of 1.4 milliseconds higher round-trip times globally compared to IPv4—but this gap varies dramatically depending on your ISP, location, and network configuration.
The Problem: This is perhaps the most impactful issue. When IPv6 is enabled but misconfigured or broken connectivity, applications attempt to use IPv6 first, wait for timeouts, and then fall back to IPv4. This delay compounds the user experience, making connections feel significantly slower.
Symptoms:
Technical Details: Your operating system may attempt to contact non-existent IPv6 DNS servers, wait for responses that never arrive, and only after multiple timeout periods will it retry with IPv4. RFC 6555 specifies timeout values, but the damage to user experience has already occurred.
The Problem: The "Happy Eyeballs" algorithm (RFC 6555 and RFC 8305) is designed to improve dual-stack connection performance by attempting both IPv4 and IPv6 simultaneously, with a slight preference for IPv6. However, the timeout values can introduce noticeable delays.
How It Works:
The Catch: If IPv6 is slow but functional, it may win the race and provide a degraded experience. Research shows that a 300ms head start gives IPv6 routes only a 1% chance of being rejected even when IPv4 routes are significantly faster. This algorithmic bias toward IPv6 can inadvertently select slower paths.
The Problem: IPv6 and IPv4 traffic often take completely different paths through the internet. While IPv4 has decades of optimized peering agreements and routing configurations, IPv6 peering relationships are still maturing.
Real-World Examples:
Why It Happens: Network operators prioritized IPv4 routing optimizations for years. IPv6 is newer, so peering agreements, BGP configurations, and traffic engineering are less mature. Your IPv6 packets might travel hundreds of miles further than your IPv4 packets to reach the same destination.
The Problem: Network hardware vendors spent decades optimizing IPv4 packet processing. Many routers, switches, and network interface cards include hardware acceleration specifically for IPv4, while IPv6 packets may be processed through slower software paths.
Technical Background:
While this gap is narrowing with modern hardware, legacy equipment still in widespread use creates performance disparities. Additionally, some routers use special shortcuts and offloading for IPv4 that don't exist for IPv6, forcing the CPU to handle all IPv6 throughput.
The Problem: IPv6 handles fragmentation differently than IPv4, and these differences create real-world performance issues.
Key Statistics:
Why It Matters:
Common Workaround: Many IPv6 servers clamp MTU to the protocol-mandated minimum of 1,280 bytes, sacrificing some performance for reliability. This conservative approach prevents fragmentation issues but reduces throughput.
The Problem: Internet Service Provider (ISP) support for IPv6 varies dramatically. Some ISPs provide native, high-quality IPv6 connectivity, while others use tunneling mechanisms that add overhead and latency.
Tunneling Overhead:
Peering Quality: Even with native IPv6, your ISP's IPv6 peering agreements may be inferior to their IPv4 peering. They may have excellent IPv4 connectivity to major content providers but route IPv6 through less optimal paths due to immature BGP relationships.
The Problem: CDNs have extensive IPv4 infrastructure with caches distributed globally. IPv6 cache presence is often limited, forcing connections to traverse longer distances.
Impact: TCP connection establishment times for IPv6 are relatively higher than IPv4 when CDN caches exist for IPv4 but are largely absent for IPv6. You might connect to a CDN edge server 10 miles away via IPv4 but 500 miles away via IPv6.
Use test-ipv6.run to measure and compare IPv4 vs IPv6 latency side-by-side. This tool:
For more detailed latency measurement techniques, see measure-ipv4-vs-ipv6-latency.md.
What to Look For:
# IPv4 traceroute
traceroute example.com
# IPv6 traceroute
traceroute6 example.com
Compare hop counts and latency at each hop. IPv6 routes with significantly more hops or higher per-hop latency indicate suboptimal peering or routing. See traceroute-ipv6 for more details.
# IPv4 MTU test (Linux/macOS)
ping -M do -s 1472 example.com
# IPv6 MTU test
ping6 -M do -s 1452 example.com
Failures at expected MTU sizes indicate fragmentation issues. If smaller packets succeed but larger ones fail, PMTUD is broken.
# Time IPv4 DNS lookup
time nslookup -type=A example.com
# Time IPv6 DNS lookup
time nslookup -type=AAAA example.com
Significant delays in AAAA record lookups suggest DNS server issues with IPv6. For more comprehensive DNS testing methods, see test-ipv6-dns-resolution.md.
Use browser developer tools (Network tab) to observe:
1. Test Your Configuration Visit test-ipv6.run to identify your specific issues. A score below 7/10 indicates problems worth addressing.
2. Check Your Router Settings
3. Optimize DNS Configuration
4. Disable IPv6 as a Last Resort If IPv6 is truly broken and causing significant delays, disabling it can improve performance by eliminating timeout delays. However, this should be temporary—contact your ISP to fix the underlying issue.
1. Implement Proper IPv6 Peering
2. Address MTU Issues
3. Upgrade Hardware
4. Monitor and Alert
5. Follow Best Practices
IPv6 slowness is almost always a symptom of immature infrastructure, not an inherent protocol limitation. As IPv6 deployment matures, performance should match or exceed IPv4:
If your IPv6 is slower, something is misconfigured or broken. Use tools like test-ipv6.run to identify the specific issue, then work with your ISP or network administrator to resolve it. Don't accept poor IPv6 performance as normal—demand better infrastructure and configuration.
The future of the internet is IPv6. Poor performance today stems from transition challenges, not fundamental protocol flaws. By identifying and fixing these issues, we can build a faster, more reliable IPv6 internet for everyone.
Test your IPv6 performance now: test-ipv6.run
Compare IPv4 vs IPv6 latency, identify broken configurations, and get a comprehensive readiness score—all directly in your browser with no backend required.