As IPv6 is gradually rolled out globally, more and more websites and services are supporting the IPv6 protocol. However, many website administrators find that access speeds are significantly lower than with IPv4 after deploying IPv6, with higher latency, and even issues such as slow DNS resolution, TCP handshake delays, and slow page loading. This phenomenon is not uncommon, and the reasons mostly stem from multiple factors including network topology, DNS configuration, server optimization, client compatibility, and application layer protocols.

　　IPv6 offers a larger address space and a simplified header design compared to IPv4, but it also has some characteristics that can affect access speed. For example, IPv6 has a fixed and larger packet header length, and its path MTU discovery mechanism is complex. If there are intermediate devices in the network link that do not support IPv6, it will trigger ICMPv6 message returns, causing connection initialization delays. Furthermore, the global deployment of IPv6 networks is not yet fully mature, and IPv6 routing at some operators or international nodes may be unstable, causing packet loss or increased RTT (Round-Trip Time). Especially in cross-border access scenarios, IPv6 routing paths may circumvent multiple autonomous systems, resulting in significantly higher latency than IPv4. The inherent complexity of network topology is one of the fundamental reasons for slow IPv6 website access.

　　DNS configuration is also a crucial factor affecting IPv6 performance. IPv6 domain name resolution relies on AAAA records. If a website's AAAA records are improperly configured, or if the DNS server's IPv6 access is unstable, it will lead to prolonged resolution time, thus impacting page loading speed. Many website administrators, when deploying IPv6, only add AAAA records but neglect the priority and resolution strategy of AAAA and A records. When clients have both IPv4 and IPv6 addresses, they may attempt an IPv6 connection but fail before falling back to IPv4. This "happy eyeballs" mechanism, while ensuring a successful connection, introduces an additional latency of 100ms to 500ms. Therefore, proper DNS configuration and optimized resolution response speed are paramount for optimizing IPv6 access performance. The following command can be used to test DNS AAAA resolution time:

dig AAAA example.com +short
dig +tcp AAAA example.com

　　Comparing the return times of different DNS nodes and selecting the service provider with the fastest response time or configuring multi-node Anycast DNS can help reduce the initial IPv6 connection latency.

　　Server-side configuration also affects IPv6 performance. First, ensure the server kernel supports and optimizes the IPv6 stack, such as the Linux kernel's TCP dual-stack strategy, kernel routing table, and Neighbor Discovery cache capacity. IPv6's Neighbor Solicitation and Router Advertisement mechanisms may increase latency in large-scale access scenarios, therefore kernel parameters need to be tuned.

sysctl -w net.ipv6.neigh.default.gc_thresh1=1024
sysctl -w net.ipv6.neigh.default.gc_thresh2=2048
sysctl -w net.ipv6.neigh.default.gc_thresh3=4096

　　Simultaneously, ensure that the firewall configuration allows necessary ICMPv6 packet types to pass through; otherwise, path MTU discovery failure and TCP connection blocking may occur. For example, allow ICMPv6 messages of types 128-137 to ensure that neighbor discovery, MTU detection, and redirection mechanisms function correctly. If the server has a load balancer or CDN deployed, ensure that its IPv6 support is complete and matches the origin server's routing optimization; otherwise, cross-node hops may increase latency.

　　Web server and application layer configurations are also important factors affecting IPv6 performance. Some web servers enable dual-stack listening by default but do not optimize parameters such as TCP backlog, KeepAlive, and congestion control, resulting in slow response times for high-concurrency IPv6 connections. This can be addressed by adjusting the `listen` parameter in Nginx or Apache, explicitly binding IPv6 addresses, and optimizing connection reuse strategies.

listen [::]:443 ipv6only=on;
keepalive_timeout 65;
tcp_nopush on;

　　Enabling HTTP/2 or QUIC protocols can also significantly reduce latency under IPv6, as these protocols support multiplexing and connection multiplexing, reducing the time required for TCP handshakes and TLS establishment.

　　Client compatibility and access policies are also crucial factors. Due to the uneven global deployment of IPv6 and significant differences in IPv6 path quality across different operators, clients may prioritize IPv6, but the actual path performance may be poor, leading to slow access. In such cases, a "Happy Eyeballs" strategy can be used to optimize client connections, quickly selecting the IPv4 or IPv6 path with the lowest latency. For server administrators, deploying CDN or Anycast IPv6 nodes can effectively improve global access speeds and reduce latency jitter.

　　Another common bottleneck is the IPv6 MTU issue. The default MTU for IPv6 is 1280 bytes. However, in real-world networks, when the physical link MTU is large, if an ICMPv6 Packet Too Big message is lost in the intermediate links, the server may fail to send fragmented packets, leading to TCP connection retransmissions. In severe cases, this can cause webpage loading delays or slow file downloads. The solution is to adjust the server's TCP MSS or use Path MTU Discovery.

ip -6 route change default mtu 1400
sysctl -w net.ipv6.tcp_mtu_probing=1

　　Enabling MTU probing allows for dynamic adaptation of network links, preventing latency caused by fragmentation failures.

　　Monitoring and analysis tools are equally indispensable for IPv6 optimization. Tools such as ping6, traceroute6, and mtr can be used to analyze packet loss rate, latency distribution, and hop count on IPv6 network paths.

ping6 -c 10 example.com
traceroute6 example.com
mtr -6 example.com

　　Data analysis can identify performance bottlenecks specific to certain carriers or intermediate nodes, allowing for targeted adjustments to routing, CDN node deployment, or optimization of BGP advertising policies. Furthermore, web performance monitoring tools (such as WebPageTest, Pingdom, and Lighthouse) support IPv6 testing, measuring key metrics like DNS lookup time, TCP/TLS establishment time, and first-screen rendering time, helping operations personnel identify IPv6 performance bottlenecks.

　　Overall, slow or high-latency IPv6 website access is typically caused by a combination of factors, including network topology, DNS configuration, server protocol stack, application layer optimization, client policies, and MTU detection. The core optimization approach is end-to-end analysis: first, ensure proper DNS configuration and rapid resolution; second, optimize the server's IPv6 stack, routing table, neighbor discovery, and MTU detection; third, optimize TCP/HTTP configuration at the web service application layer; and finally, improve the global access experience through CDN, Anycast, and Happy Eyeballs policies. Continuously monitoring and analyzing network path, packet loss, and latency changes are also crucial for IPv6 performance optimization. Through scientific and systematic optimization methods, IPv6 website access latency can be significantly reduced, user access speed and service stability can be improved, and the true value of IPv6 deployment can be realized.