What is HTTP/3 QUIC and how does it improve performance?

HTTP/3 uses the QUIC transport protocol (built on UDP) instead of TCP, addressing fundamental TCP limitations. TCP head-of-line blocking: HTTP/2 multiplexes multiple streams over one TCP connection. But TCP is ordered — a lost packet blocks ALL streams until it's retransmitted. On lossy networks (mobile, WiFi), this causes stalls. QUIC streams are independent — a lost packet only blocks the affected stream. Faster connection establishment: TCP requires 2 round trips (SYN, SYN-ACK, ACK + TLS 1.3 requires additional RTTs). QUIC combines transport + TLS in 1 or 0 RTTs for repeat connections. Sub-50ms connection times vs 150ms+ for TCP+TLS. Connection migration: QUIC connections survive IP address changes using connection IDs — switching from WiFi to cellular doesn't reset the connection. Improved congestion control: QUIC implements congestion control per-stream with more sophisticated algorithms than TCP's single-stream model. Adoption: supported by all major CDNs (Cloudflare, Fastly, CloudFront). Chrome, Firefox, Edge, and Safari support HTTP/3. Enable in Nginx with ngx_http_v3_module. Most beneficial for users on high-latency or lossy connections — mobile users in areas with poor connectivity see the most improvement.