For most websites serving assets over HTTP/2 or HTTP/3, adding multiple CDN subdomains is not a dependable way to reduce load time. HTTP/2 can multiplex many requests over one connection, so the old reason for splitting assets across hostnames is largely gone; extra hostnames can add DNS and connection setup work. Keep one asset hostname as the default, improve CDN caching and delivery first, and retain sharding only if tests on your own site show a repeatable benefit.
What multiple CDN subdomains are meant to do
Domain sharding means distributing assets across hostnames—for example, serving images from img1.example.com and img2.example.com rather than one asset hostname. Under older HTTP/1.1 browser behavior, per-domain connection limits could restrict how many downloads ran at once. Splitting assets across hostnames could open more connections and allow more downloads to proceed in parallel.
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That historical workaround is not the same as using a CDN. A CDN can serve eligible, cached content from an edge location nearer a visitor and reduce trips to your origin. Those benefits depend on routing, cache rules, and whether the content is actually cached—not on how many subdomains you create.
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Why sharding is usually not a modern speed fix
HTTP/2 already multiplexes requests
HTTP/2 allows multiple requests to share one connection, reducing the reason to spread them across hostnames. The U.S. Web Design System’s HTTP/2 Performance Guide advises: “Do not use the domain splitting technique.” For a site whose asset requests negotiate HTTP/2, starting with one asset origin is usually the simpler choice.
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Each hostname has setup costs
A new hostname may require DNS resolution and, when it establishes a separate transport connection, TCP and TLS setup. These steps consume time. Multiple connections can also make it harder to prioritize streams together across a page’s resources. Consequently, any concurrency gained from sharding can be offset by connection overhead.
HTTP/3 is worth evaluating, not assuming
Check the protocol actually negotiated for the relevant asset requests; do not infer it from a CDN’s marketing or configuration alone. Google Cloud’s CDN guidance recommends enabling HTTP/3 where supported, but that is an option to evaluate rather than a guaranteed improvement for every site. HTTP/3 does not make extra hostnames automatically faster.
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Improve CDN delivery before adding subdomains
- Route the asset hostname through the CDN. A hostname by itself does not provide edge caching. For example, Cloudflare says DNS-only, unproxied records are not cached by its service, and its troubleshooting guidance notes that subdomains outside its CDN do not receive those caching benefits.
- Check cache eligibility and behavior. Verify that the assets can be cached, that the relevant cache rules apply, and that requests are getting cache hits. A CDN cannot serve an edge copy that is ineligible or has not been cached.
- Consider cache architecture. Edge or tiered caching can reduce origin distance and repeated origin work. Google Cloud also describes request coalescing, which can limit concurrent cache-fill requests to an origin. These are CDN design choices, separate from domain sharding.
- Measure page-level results. Compare representative pages, visitor locations, and network conditions. Look at page load outcomes alongside DNS, connection setup, and cache behavior; a change in one request’s timing does not necessarily improve the full page.
How to test whether sharding helps your site
- Establish the baseline. Record representative page-load results with the current asset hostname and CDN configuration. Include the audience geographies and network conditions that matter to your site.
- Confirm negotiated protocols. Inspect the asset requests in the browser’s network tools and confirm whether they use HTTP/1.1, HTTP/2, or HTTP/3. Protocol behavior can differ by browser, CDN configuration, and request path.
- Verify CDN and cache coverage. Confirm each proposed hostname is correctly routed through the CDN, covered by valid TLS configuration, and eligible for the intended cache rules. An unproxied or otherwise out-of-CDN subdomain may not receive the CDN’s caching benefits.
- Run a controlled comparison. Compare the same representative pages with the existing arrangement and the proposed shard arrangement. Keep other relevant settings constant, and assess repeatability rather than relying on one run.
- Keep sharding only if the measured gain matters. Retain the additional hostnames only when the improvement repeats for the audience that matters and justifies the added configuration and maintenance. There is no universal threshold established for every site.
When sharding might still be worth testing
A 2017 Akamai paper, Domain-Sharding for Faster HTTP/2 in Lossy Cellular Networks, reports a 12% median page-load-time improvement in good network conditions for its tested HTTP/2 sharding setup. This is a result from that paper’s particular experiment, not a general forecast for other sites. The paper extract also cautions that pages with hundreds of small objects can lose the benefit because multiple connections add TCP/TLS setup latency. Treat the result as a reason to test a relevant setup, not as a promise.
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- The new hostname does not appear to cache: check whether its DNS record is proxied/routed through the CDN and whether the asset qualifies under the cache rules. A DNS record alone does not mean CDN caching is active.
- Sharding makes the page slower: inspect whether extra DNS resolution or transport handshakes outweigh any parallelism. Compare page-level results and consider reverting to one asset hostname.
- Different users see different results: verify the negotiated protocol and repeat the comparison across representative geographies and network conditions. A configuration’s result need not be uniform for every visitor.
- A hostname has TLS or delivery issues: confirm it is covered by valid TLS and correctly routed through the CDN before comparing speed; otherwise the test is not comparing equivalent, working delivery paths.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Frequently Asked Questions
Does adding a CDN subdomain automatically make assets faster?
No. The hostname must be routed through the CDN, and the requested content must be eligible for caching and served from cache for edge caching to help.
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Should I delete existing asset subdomains?
Not without checking dependencies and measuring. Confirm routing, TLS, cache behavior, and page performance before changing a live asset setup.
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