Preconnected

Preconnected is an adjective used to describe a state in which network connections to a particular host have been established before they are needed. In computing, preconnected connections are opened in advance to reduce the latency of future requests. This is commonly used in web performance to speed up loading of third‑party resources by performing the initial steps of the connection setup—DNS resolution, TCP handshake, and, if applicable, the TLS handshake—before the browser actually requests a resource from the origin.

Implementation commonly relies on the HTML link rel="preconnect" hint, which instructs the browser to establish early connections to the specified origins. The browser may also reuse these preconnected sockets for subsequent requests, which can shorten the critical path for the first data transfer. Preconnected connections are typically maintained for a limited time and may be closed if not used, to conserve resources.

Benefits include reduced latency for early assets such as fonts, analytics, or CDNs, and potentially faster rendering of the page. Drawbacks include wasted network and server resources when the preconnected host is not used, increased privacy exposure by revealing a user’s intent, and the risk of overusing hints leading to diminished gains.

Practical guidance: apply preconnect selectively to origins that are immediately needed during initial page load, and avoid overuse. Use crossOrigin where required. Assess impact with performance tooling such as Lighthouse or WebPageTest; measure changes in time-to-first-byte, start render, and overall page speed.