PERFORMANCE · SEO GLOSSARY
DNS Prefetch (SEO)
The `<link rel="dns-prefetch">` resource hint that instructs the browser to perform DNS resolution for a specified origin in advance — reducing the DNS lookup latency for third-party origins that aren\'t critical enough to warrant a full preconnect but still contribute to page load time.
Definition
DNS prefetch is a browser resource hint (`<link rel="dns-prefetch" href="https://third-party.example">`) that triggers an early DNS lookup for a third-party origin. DNS resolution — converting a domain name to an IP address — typically takes 20–120ms on average networks. When a page uses many third-party resources (analytics, advertising, social sharing, comment systems, fonts, CDN assets), each distinct origin requires its own DNS lookup before the first request can be made to it. DNS prefetch allows these lookups to happen early in the page load process, reducing the waiting time for subsequent resource requests. DNS prefetch vs preconnect: DNS prefetch only performs DNS resolution (the cheapest connection step); preconnect performs DNS + TCP handshake + TLS negotiation (more expensive, more beneficial). For critical-path origins (those needed early in the render): use preconnect. For secondary origins (analytics, social widgets, comment systems that load after the page renders): use dns-prefetch. Implementation: `<link rel="dns-prefetch" href="//analytics.example.com">` in the `<head>`. Works for any origin the browser doesn\'t already have a cached DNS record for. Multiple dns-prefetch hints are acceptable (unlike preconnect, which has higher cost per entry). Browser prefetch cache: DNS records are typically cached by the browser for 60 seconds. For pages with high session duration (users spend several minutes before navigating to the next page), DNS prefetch on internal navigation targets can also reduce the latency of navigating between pages.
Why it matters for SEO
DNS prefetch is a low-cost optimisation applicable to any page using multiple third-party origins. For pages with 5–15 third-party resource origins (a common number on marketing and content pages with full analytics and advertising stacks), adding dns-prefetch hints for all non-critical origins can reduce the cumulative DNS resolution overhead that contributes to page load delay. While individual impact per origin is modest (20–120ms), the cumulative effect of removing DNS latency across multiple origins adds up.
How DeepSEOAnalysis checks this
DeepSEOAnalysis identifies third-party origins referenced by the audited page and checks for dns-prefetch hints. Missing dns-prefetch hints for detected third-party origins are flagged as optimisation opportunities. The audit reports which third-party origins are present without dns-prefetch hints, allowing prioritisation of which ones would benefit most from early DNS resolution.
GLOSSARY
Related terms
performance
Preconnect (SEO)
The `<link rel="preconnect">` resource hint that instructs the browser to establish a TCP+TLS connection to a specified origin in advance of it being needed — reducing latency for third-party resources (fonts, CDN assets, analytics) that would otherwise wait for connection establishment when first requested.
Read definition →performance
Resource Hints
HTML link elements that instruct browsers to prefetch, preconnect, preload, or prerender resources — used to improve page loading performance by initiating connections and resource downloads earlier than they would otherwise begin, reducing the impact of network latency on LCP and other Core Web Vitals.
Read definition →performance
Render-Blocking Resources (SEO)
CSS stylesheets and JavaScript files that block the browser from rendering page content until they are downloaded and processed — a primary cause of poor LCP and FCP (First Contentful Paint) scores, requiring deferral, async loading, or inline critical CSS strategies to resolve.
Read definition →performance
Performance Budget
A predefined set of constraints on web page performance metrics — such as maximum JavaScript bundle size, maximum LCP time, or maximum total blocking time — used to prevent performance regression as a website evolves, by alerting or failing builds when the page\'s performance metrics would exceed the defined thresholds.
Read definition →See how your site scores on DNS Prefetch (SEO).
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