The tech lead checks "Enable HTTP/3" in the CDN. Lighthouse gains three points. Server TTFB did not move: 890 ms on the cart page. Management concludes "hosting is slow". In reality, HTTP/3 mostly optimises transport — not PHP generation or an overloaded database.
HTTP/3 runs on QUIC (UDP) rather than TCP. It cuts handshake round trips, handles packet loss better on mobile and network changes (Wi‑Fi → 4G). A useful evolution — not a magic wand for uncached WordPress.
What HTTP/3 improves — and what it ignores
| Layer | HTTP/3 helps? | Example |
|---|---|---|
| TLS handshake + transport | Yes | Faster connect on unstable 4G |
| Request multiplexing | Yes (like h2) | Many small assets |
| PHP / SQL TTFB | No | Slow dynamic page |
| Image / JS weight | No | High LCP |
| Slow DNS | No | Resolution before QUIC |
On a static CDN-served site with many short connections, HTTP/3 may shave 50–150 ms perceived latency. On a slow origin admin or heavy POST API, effect is invisible.
When enabling makes sense
Yes, enable if:
- Origin TTFB already acceptable (< 400 ms dynamic or effective cache).
- Significant mobile traffic, international audience.
- CDN handles QUIC without complex server config.
- You measure with RUM before/after.
Defer if:
- OPcache off, no page cache, slow SQL.
- You have not fixed Core Web Vitals LCP/CLS yet.
- Host advertises HTTP/3 but forces a misconfigured proxy (loops, bad headers).
Typical deployment: CDN vs origin
Most HTTP/3 sites use a CDN:
- Visitor ↔ edge QUIC (HTTP/3)
- Edge ↔ origin often still HTTP/1.1 or HTTP/2
The host "supports HTTP/3" via CDN partner, not necessarily bare VPS. Check contract: TLS termination where, certificates, cache rules.
Minimal test:
curl --http3-only -sI https://example.com | head -5
Compare --http2 and --http1.1 on the same static asset.
The peak: optimising protocol before application
CDN marketing loves "HTTP/3 ready". It does not replace OPcache, SQL indexes or image compression.
Decide and move forward without blind spots
- Measure origin TTFB and current LCP.
- Fix PHP, cache, assets if TTFB > 600 ms or LCP red.
- Enable HTTP/3 via CDN; compare mobile RUM over 7 days.
- Document edge vs origin protocol for support.
Compare compatible hosts and CDNs via the directory and comparator.
Frequently asked questions
Is HTTP/3 mandatory in 2026?
No — prioritise TTFB and content before QUIC if the server is the bottleneck.
Shared host offers HTTP/3 — enable?
Yes as risk-free toggle with measurement; no if it avoids real origin diagnosis.
HTTP/3 without CDN?
Possible at origin (Nginx/Caddy), but rare in practice — most use CDN edge.
How to test HTTP/3?
DevTools Protocol h3 or curl --http3-only; compare to h2.
Before celebrating HTTP/3, one question: is origin TTFB already green? Otherwise you mostly speed up a PHP queue — not the network.