# Performance

Out of the box, SvelteKit does a lot of work to make your applications as performant as possible:

- Code-splitting, so that only the code you need for the current page is loaded
- Asset preloading, so that 'waterfalls' (of files requesting other files) are prevented
- File hashing, so that your assets can be cached forever
- Request coalescing, so that data fetched from separate server `load` functions is grouped into a single HTTP request
- Parallel loading, so that separate universal `load` functions fetch data simultaneously
- Data inlining, so that requests made with `fetch` during server rendering can be replayed in the browser without issuing a new request
- Conservative invalidation, so that `load` functions are only re-run when necessary
- Prerendering (configurable on a per-route basis, if necessary) so that pages without dynamic data can be served instantaneously
- Link preloading, so that data and code requirements for a client-side navigation are eagerly anticipated

Nevertheless, we can't (yet) eliminate all sources of slowness. To eke out maximum performance, you should be mindful of the following tips.

## Diagnosing issues

Google's [PageSpeed Insights](https://pagespeed.web.dev/) and (for more advanced analysis) [WebPageTest](https://www.webpagetest.org/) are excellent ways to understand the performance characteristics of a site that is already deployed to the internet.

Your browser also includes useful developer tools for analysing your site, whether deployed or running locally:

* Chrome - [Lighthouse](https://developer.chrome.com/docs/lighthouse/overview#devtools), [Network](https://developer.chrome.com/docs/devtools/network), and [Performance](https://developer.chrome.com/docs/devtools/performance) devtools
* Edge - [Lighthouse](https://learn.microsoft.com/en-us/microsoft-edge/devtools-guide-chromium/lighthouse/lighthouse-tool), [Network](https://learn.microsoft.com/en-us/microsoft-edge/devtools-guide-chromium/network/), and [Performance](https://learn.microsoft.com/en-us/microsoft-edge/devtools-guide-chromium/evaluate-performance/) devtools
* Firefox - [Network](https://firefox-source-docs.mozilla.org/devtools-user/network_monitor/) and [Performance](https://hacks.mozilla.org/2022/03/performance-tool-in-firefox-devtools-reloaded/) devtools
* Safari - [enhancing the performance of your webpage](https://developer.apple.com/library/archive/documentation/NetworkingInternetWeb/Conceptual/Web_Inspector_Tutorial/EnhancingyourWebpagesPerformance/EnhancingyourWebpagesPerformance.html)

Note that your site running locally in `dev` mode will exhibit different behaviour than your production app, so you should do performance testing in [preview](building-your-app#Preview-your-app) mode after building.

### Instrumenting

If you see in the network tab of your browser that an API call is taking a long time and you'd like to understand why, you may consider instrumenting your backend with a tool like [OpenTelemetry](https://opentelemetry.io/) or [Server-Timing headers](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Server-Timing).

## Optimizing assets

### Images

Reducing the size of image files is often one of the most impactful changes you can make to a site's performance. Svelte provides the `@sveltejs/enhanced-img` package, detailed on the [images](images) page, for making this easier. Additionally, Lighthouse is useful for identifying the worst offenders.

### Videos

Video files can be very large, so extra care should be taken to ensure that they're optimized:

- Compress videos with tools such as [Handbrake](https://handbrake.fr/). Consider converting the videos to web-friendly formats such as `.webm` or `.mp4`.
- You can [lazy-load videos](https://web.dev/articles/lazy-loading-video) located below the fold with `preload="none"` (though note that this will slow down playback when the user _does_ initiate it).
- Strip the audio track out of muted videos using a tool like [FFmpeg](https://ffmpeg.org/).

### Fonts

SvelteKit automatically preloads critical `.js` and `.css` files when the user visits a page, but it does _not_ preload fonts by default, since this may cause unnecessary files (such as font weights that are referenced by your CSS but not actually used on the current page) to be downloaded. Having said that, preloading fonts correctly can make a big difference to how fast your site feels. In your [`handle`](hooks#Server-hooks-handle) hook, you can call `resolve` with a `preload` filter that includes your fonts.

You can reduce the size of font files by [subsetting](https://web.dev/learn/performance/optimize-web-fonts#subset_your_web_fonts) your fonts.

## Reducing code size

### Svelte version

We recommend running the latest version of Svelte. Svelte 5 is smaller and faster than Svelte 4, which is smaller and faster than Svelte 3.

### Packages

[`rollup-plugin-visualizer`](https://www.npmjs.com/package/rollup-plugin-visualizer) can be helpful for identifying which packages are contributing the most to the size of your site. You may also find opportunities to remove code by manually inspecting the build output (use `build: { minify: false }` in your [Vite config](https://vitejs.dev/config/build-options.html#build-minify) to make the output readable, but remember to undo that before deploying your app), or via the network tab of your browser's devtools.

### External scripts

Try to minimize the number of third-party scripts running in the browser. For example, instead of using JavaScript-based analytics consider using server-side implementations, such as those offered by many platforms with SvelteKit adapters including [Cloudflare](https://www.cloudflare.com/web-analytics/), [Netlify](https://docs.netlify.com/monitor-sites/site-analytics/), and [Vercel](https://vercel.com/docs/analytics).

To run third party scripts in a web worker (which avoids blocking the main thread), use [Partytown's SvelteKit integration](https://partytown.builder.io/sveltekit).

### Selective loading

Code imported with static `import` declarations will be automatically bundled with the rest of your page. If there is a piece of code you need only when some condition is met, use the dynamic `import(...)` form to selectively lazy-load the component.

## Navigation

### Preloading

You can speed up client-side navigations by eagerly preloading the necessary code and data, using [link options](link-options). This is configured by default on the `<body>` element when you create a new SvelteKit app.

### Non-essential data

For slow-loading data that isn't needed immediately, the object returned from your `load` function can contain promises rather than the data itself. For server `load` functions, this will cause the data to [stream](load#Streaming-with-promises) in after the navigation (or initial page load).

### Preventing waterfalls

One of the biggest performance killers is what is referred to as a _waterfall_, which is a series of requests that is made sequentially. This can happen on the server or in the browser, but is especially costly when dealing with data that has to travel further or across slower networks, such as a mobile user making a call to a distant server.

In the browser, waterfalls can occur when your HTML kicks off request chains such as requesting JS which requests CSS which requests a background image and web font. SvelteKit will largely solve this class of problems for you by adding [`modulepreload`](https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel/modulepreload) tags or headers, but you should view [the network tab in your devtools](#Diagnosing-issues) to check whether additional resources need to be preloaded.
- Pay special attention to this if you use [web fonts](#Optimizing-assets-Fonts) since they need to be handled manually.
- Enabling [single page app (SPA) mode](single-page-apps) will cause such waterfalls. With SPA mode, an empty page is generated, which fetches JavaScript, which ultimately loads and renders the page. This results in extra network round trips before a single pixel can be displayed.

Waterfalls can also occur on calls to the backend whether made from the browser or server. E.g. if a universal `load` function makes an API call to fetch the current user, then uses the details from that response to fetch a list of saved items, and then uses _that_ response to fetch the details for each item, the browser will end up making multiple sequential requests. This is deadly for performance, especially for users that are physically located far from your backend.
- Avoid this issue by using [server `load` functions](load#Universal-vs-server) to make requests to backend services that are dependencies from the server rather than from the browser. Note, however, that server `load` functions are also not immune to waterfalls (though they are much less costly since they rarely involve round trips with high latency). For example, if you query a database to get the current user and then use that data to make a second query for a list of saved items, it will typically be more performant to issue a single query with a database join.

## Hosting

Your frontend should be located in the same data center as your backend to minimize latency. For sites with no central backend, many SvelteKit adapters support deploying to the _edge_, which means handling each user's requests from a nearby server. This can reduce load times significantly. Some adapters even support [configuring deployment on a per-route basis](page-options#config). You should also consider serving images from a CDN (which are typically edge networks) — the hosts for many SvelteKit adapters will do this automatically.

Ensure your host uses HTTP/2 or newer. Vite's code splitting creates numerous small files for improved cacheability, which results in excellent performance, but this does assume that your files can be loaded in parallel with HTTP/2.

## Further reading

For the most part, building a performant SvelteKit app is the same as building any performant web app. You should be able to apply information from general performance resources such as [Core Web Vitals](https://web.dev/explore/learn-core-web-vitals) to any web experience you build.