How do you implement routing, dynamic imports, and lazy loading in Svelte?

Long Answer

Optimizing a Svelte project for Core Web Vitals starts with a routing plan that keeps the initial payload small and shifts non-critical work off the critical path. The pillars are route-based code-splitting, component-level dynamic imports, and pragmatic lazy loading for media and data, all wrapped in a UX that avoids layout shifts and interaction delays.

1) Routing as the performance backbone
With SvelteKit, the filesystem is your router. Each src/routes/... entry becomes its own chunk boundary, so navigating from / to /dashboard only fetches what is needed. Group feature areas under route segments (for example, /app/(marketing), /app/(auth), /app/(console)), allowing independent caching policies. Use shallow layouts (+layout.svelte, +layout.ts) to share minimal chrome and avoid pulling heavy providers into every page. Keep global layout light; push expensive logic into child routes that load later.

2) Route-level code-splitting and dynamic imports
Route files split automatically, but you can go further by lazy loading expensive widgets inside a page. Wrap non-critical components behind await import('./HeavyChart.svelte') and render them via an awaited promise or a small loader component. This technique defers parsing and execution until the UI is ready or a user interacts with a tab or accordion. For rare admin tools, load the entire feature on demand from a separate route or guard it behind a role-checked dynamic import. Keep the top of each route free of heavy import statements to avoid bundling them into the initial chunk.

3) Smart prefetching and link hydration
SvelteKit supports data-sveltekit-preload-data="hover" so that hovering a link prefetches JSON and modules just before a click. Use this for high-probability navigation paths like “Next” or “Checkout.” Prefer “intent-based” prefetch on viewport for primary CTAs, but do not blanket-prefetch every link, which can turn idle time into bandwidth waste. Combine with small HTTP cache TTLs for route data to make back-and-forth interactions feel instant without bloating initial load.

4) Media and font lazy loading without CLS
For images, set explicit width/height or aspect-ratio containers so the browser can reserve space and prevent CLS. Use <img loading="lazy" decoding="async">, responsive srcset/sizes, and modern formats (AVIF, WebP), with a tiny placeholder (blurred low-quality image) inside a fixed box. Defer non-critical web fonts with font-display: swap and preconnect to font origins; consider self-hosting to avoid third-party delays. For icons, prefer SVG sprites or inline SVG over icon fonts to cut blocking requests.

5) Data fetching, streaming, and partial hydration
In +page.ts, keep load() lean, return only the data needed for above-the-fold content, and fetch secondary panels after mount. Use await parent() sparingly to avoid coupling pages to heavy layout data. For long payloads, stream responses from endpoints or chunk lists with infinite scroll and intersection observers. Pair these with skeleton components that match final dimensions to avoid jumps.

6) Interaction latency and script budgeting
Core Web Vitals penalize heavy main-thread work. Cap total JS per route by deferring editor widgets, charts, and WYSIWYGs until interaction. Avoid large all-in-one UI libraries; prefer small Svelte components or tree-shaken ESM modules. Debounce resize/scroll handlers, and move polling to web workers when reasonable. Audit bundle size regularly; if a dependency is large, isolate it behind a dynamic import so it only lands where needed.

7) Caching and HTTP hints
Emit HTTP caching headers per route: long-cache immutable assets for hashed chunks, short TTL for JSON data, and stale-while-revalidate for feed-like endpoints. Add <link rel="preload"> for the hero image and first route chunk, plus <link rel="preconnect"> to critical origins. Keep preloads selective; preload abuse can crowd out the critical CSS/JS.

8) Testing and budgets for Core Web Vitals
Automate Lighthouse or WebPageTest in CI for key routes. Track LCP element, INP/FID, and CLS contributors. Set performance budgets: maximum initial JS, maximum image bytes on /, and an LCP target under 2.5s on 4G. Fail builds when a route exceeds limits. Add synthetic checks that navigate across routes to confirm that dynamic imports resolve quickly and that skeletons prevent layout shifts.

9) Progressive enhancement and resilience
Ensure content and navigation remain usable if a dynamic import fails or JS is delayed. Provide SSR for above-the-fold HTML and keep critical interactions accessible. Lazy modules must be optional enhancements; where core functionality depends on them, show a deterministic loader with clear state and keep dimensions fixed.

By combining route-based splitting, component-level dynamic imports, and disciplined lazy loading, a Svelte app ships less JavaScript on first paint, improves LCP and INP, and avoids CLS. This keeps the experience responsive while letting advanced features arrive just in time.

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Table

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Common Mistakes

Importing heavy modules at the top of a route so they land in the initial bundle. Using global layouts to mount charts, editors, or analytics for all pages. Prefetching every link and exhausting bandwidth and CPU during idle. Lazy loading images without width/height, causing CLS. Fetching all data in load() even for below-the-fold sections. Shipping large icon fonts and blocking LCP. Letting a dynamic import gate core navigation without a skeleton or fixed container. Ignoring performance budgets so bundles creep upward. Forgetting to track route-change timings and not measuring INP regressions from heavy hydration.

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Sample Answers (Junior / Mid / Senior)

Junior:
“I rely on SvelteKit’s route-based splitting and keep the global layout light. I lazy load big components with await import(...) and use <img loading="lazy"> with fixed sizes to prevent CLS. I enable hover prefetch for common links so navigation feels instant.”

Mid:
“I split features into route groups and defer charts or editors behind dynamic imports triggered by tab clicks. load() returns only above-the-fold data; secondary panels fetch after mount with skeletons that reserve space. I self-host fonts with font-display: swap and add preconnect. CI runs Lighthouse budgets per route.”

Senior:
“I design a performance budget per route, enforce SSR for the critical path, and isolate heavy dependencies behind lazy boundaries. I use intent-based prefetch, immutable caching for hashed chunks, and stale-while-revalidate for JSON. We track LCP element, INP, and CLS in CI and block merges on regressions. Fallback UI ensures progressive enhancement if a dynamic import fails.”

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Evaluation Criteria

Look for a routing plan that minimizes the global layout footprint, explicit dynamic imports for heavy features, and disciplined lazy loading of media and data with fixed dimensions and skeletons. Strong answers mention intent-based prefetch, cache headers, self-hosted fonts with font-display: swap, and CI performance budgets focused on LCP, INP, and CLS. Red flags include importing heavy modules at route top level, global layouts binding large dependencies to all pages, lazy images without dimensions, prefetching everything indiscriminately, and ignoring performance regression testing.

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Preparation Tips

Create a SvelteKit demo with / (marketing) and /app/dashboard. Keep +layout.svelte minimal. On the dashboard, gate a chart component behind a tab and load it with await import('./Chart.svelte'). Add hover prefetch on the “Dashboard” link. For images, add fixed aspect-ratio containers and LQIP placeholders. In +page.ts, fetch only hero data; defer the rest with a store updated after mount. Self-host a variable font with font-display: swap and preconnect. Add caching headers for chunks and short-TTL JSON. Wire a CI step running Lighthouse; set budgets for initial JS and image bytes, and fail builds on LCP or CLS regressions.

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Real-world Context

A startup’s Svelte landing page loaded a charting library in the root layout, bloating every route. By moving charts behind a dashboard route and using dynamic imports, initial JS dropped by 35% and LCP improved by 400 ms. Another team lazy loaded all gallery images but forgot dimensions; CLS spiked until they added aspect-ratio boxes and LQIP placeholders. A SaaS product enabled hover prefetch on primary CTAs and trimmed load() to above-the-fold data; navigation felt instant while bandwidth stayed controlled. With CI budgets on LCP and JS weight, regressions were caught before release, stabilizing Core Web Vitals.

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Key Takeaways

• Keep global layout slim; let routes own their weight with code-splitting.
  
• Use dynamic imports for heavy, below-the-fold features and admin tools.
  
• Lazy load media and data with fixed dimensions and skeletons to avoid CLS.
  
• Prefetch by intent, cache smartly, and self-host fonts with font-display: swap.
  
• Enforce performance budgets and test LCP, INP, and CLS in CI to prevent regressions.
  
  

Practice Exercise

Scenario:
Your Svelte app has a slow home page and sluggish navigation to /app/dashboard. Initial JS is heavy, images shift layout, and deploys sometimes regress Core Web Vitals.

Tasks:

1. Split features into routes: keep +layout.svelte minimal; move heavy components into child routes.
   
2. On the dashboard, place charts behind a tab and import them dynamically on first reveal. Provide a skeleton with fixed height so the layout does not shift.
   
3. Add data-sveltekit-preload-data="hover" to primary nav links. Prefetch only likely paths.
   
4. Convert images to responsive AVIF/WebP, add explicit dimensions or aspect-ratio containers, and lazy load below-the-fold media with LQIP placeholders.
   
5. Refactor +page.ts to return only above-the-fold data; fetch secondary data after mount and stream list items incrementally.
   
6. Self-host fonts, add font-display: swap, and preconnect to critical origins.
   
7. Configure cache headers: immutable for hashed chunks, short TTL with stale-while-revalidate for JSON.
   
8. Add Lighthouse in CI with budgets for initial JS and LCP; fail builds on CLS or INP regressions.
   
   

Deliverable:
A refactored Svelte app demonstrating route-based code-splitting, targeted dynamic imports, disciplined lazy loading, and measurable improvements to Core Web Vitals with automated regression guards.