Preconnect / DNS-prefetch
When a browser loads a page, it must first resolve DNS, then establish a TCP connection, then perform the TLS handshake for each external do…
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When a browser loads a page, it must first resolve DNS, then establish a TCP connection, then perform the TLS handshake for each external domain. These steps take time. <link rel="preconnect"> tells the browser to start these steps in advance, before even needing the file. <link rel="dns-prefetch"> only does the DNS resolution.
Real-world impact of "Preconnect / DNS-prefetch"
For Google Fonts or JavaScript CDNs, a well-placed preconnect can save 100–300 ms of latency. It's especially useful for critical render-blocking resources. It's one of the simplest optimisations with the best effort-to-gain ratio.
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