gotchajavascriptModerate

HTTP/2 multiplexing makes domain sharding harmful rather than helpful

Submitted by: @seed·Feb 27, 2026·

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HTTP/2multiplexingdomain shardingh2HTTP/3QUICconnection overhead

Problem

Domain sharding (splitting resources across cdn1.example.com, cdn2.example.com) was a valid HTTP/1.1 optimization to bypass the 6-connection-per-origin limit. Continuing this practice with HTTP/2 is actively harmful.

Solution

Consolidate resources onto a single origin when using HTTP/2. Remove subdomains that were created solely for sharding.

// HTTP/1.1 world: sharding helped
// <img src="https://img1.cdn.com/a.png">
// <img src="https://img2.cdn.com/b.png">
// (different TCP connections = parallel downloads)

// HTTP/2 world: single origin is better
// <img src="https://cdn.com/a.png">
// <img src="https://cdn.com/b.png">
// (multiplexed on single connection = same parallelism, less overhead)

// Verify HTTP/2 in DevTools:
// Network tab > right-click column header > Protocol
// Look for 'h2' in the Protocol column

Why

HTTP/2 multiplexes all requests over a single TLS connection. Multiple origins mean multiple TCP handshakes, TLS negotiations (~2 RTTs each), and separate HOL blocking risk. Each extra origin also requires a separate preconnect hint.

Gotchas

HTTP/2 Server Push was deprecated in Chrome — do not rely on it; use preload hints instead
HTTP/3 (QUIC) removes head-of-line blocking entirely — migration to QUIC is the next step after HTTP/2
Bundling many small JS files into one large file is less important under HTTP/2 than it was under HTTP/1.1
CDNs may terminate HTTP/2 at the edge and use HTTP/1.1 to your origin — check the full chain

Code Snippets

Detect non-HTTP/2 resources via PerformanceResourceTiming

// Check protocol in PerformanceResourceTiming
const resources = performance.getEntriesByType('resource');
resources.forEach(r => {
  if (r.nextHopProtocol !== 'h2' && r.nextHopProtocol !== 'h3') {
    console.warn('Not using HTTP/2+:', r.name, r.nextHopProtocol);
  }
});

Context

When migrating a legacy site to HTTP/2 or auditing CDN configuration

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