What is Hydration Cost?
The JavaScript work that turns server-rendered HTML into an interactive app. Hydration replays component logic in the browser, so oversized bundles delay input readiness even when paint looks instant.
Related terms
CSR Shell
A minimal HTML shell that renders content in the browser after JavaScript loads. Client rendering suits authenticated dashboards behind login, where SEO matters less and interactivity starts only after scripts execute.
Code-Splitting by Route
Automatic chunking of JavaScript per route so visitors download only what the current page needs. SvelteKit splits at route boundaries by default, keeping dashboard code out of marketing pages and initial loads lean.
Page Speed
How fast a page becomes usable, measured by field metrics like LCP and INP rather than single lab scores. Small gains compound commercially: a tenth of a second has moved retail conversions measurably.
SSR Per Request
Fresh HTML rendered on the server for every request, used for personalized or frequently changing pages. Server rendering guarantees complete markup on first paint but costs compute per visit, so reserve it for dynamic routes.
SvelteKit adapter-static
A SvelteKit build target that emits a fully static site with no server runtime. Fully static output fits documentation and marketing pages, but any route needing server logic must switch back to a server adapter.
Immutable Hashed Assets
Static files with content hashes in their filenames, emitted under the immutable build output. When code changes the hash changes, so browsers safely cache each version forever without risking stale JavaScript or styles.