Lazy Loading

Lazy loading defers fetching a resource — a script, an image, a component, a font — until something establishes that it is needed. It is the most reliable way to reduce what a page costs on first load, because the cheapest byte is the one never requested.

Common forms include images and iframes deferred until near the viewport (now a native HTML attribute), components loaded on interaction or on visibility, and route code fetched when a route is entered. On the server side the equivalent is deferring a slow data fetch behind a streaming boundary so the shell renders immediately.

Two hazards recur. Deferral changes timing, and timing is observable. A script that only loads after a genuine user gesture will not load in an automated test that dispatches a synthetic event at an element rather than at the document — a real class of test that passes locally and fails in headless. Deferral can be undone by accident, since a single eager import elsewhere in the graph can pull the deferred module back into the initial chunk. See Module Graph.

Lazy loading interacts with perceived performance rather than just measured performance: something loading late is only an improvement if the page is usable meanwhile, which is what skeleton states and streaming boundaries provide.