Page options (SSR, CSR, prerender) in Svelte - Performance & Optimization

Performance: Page options (SSR, CSR, prerender)

HIGH IMPACT

This affects how fast the main content appears and how quickly users can interact with the page.

Rendering a page with dynamic data

Svelte

export const ssr = true;

<script context="module">
  export async function load() {
    const res = await fetch('/api/data');
    const data = await res.json();
    return { props: { data } };
  }
</script>

<script>
  export let data;
</script>

<main>{JSON.stringify(data)}</main>

Server renders page with data, so main content appears immediately, improving LCP and INP.

📈 Performance GainReduces LCP by 1-3 seconds and improves interactivity as content is ready on first paint.

Rendering a page with dynamic data

Svelte

export const ssr = false;

<script>
  import { onMount } from 'svelte';
  // fetch data on client only
  let data;
  onMount(async () => {
    const res = await fetch('/api/data');
    data = await res.json();
  });
</script>

<main>{data ? JSON.stringify(data) : 'Loading...'}</main>

Disables SSR causing blank page until client fetch completes, delaying LCP and increasing INP.

📉 Performance CostBlocks LCP until client JS loads and fetch completes, increasing time by 1-3 seconds on slow networks.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
CSR only	Minimal initial DOM, built on client	Multiple reflows as content loads	High paint cost on load	[X] Bad
SSR	Full DOM sent from server	Single reflow on load	Lower paint cost	[OK] Good
Prerender	Static full DOM	Single reflow on load	Lowest paint cost	[OK] Good

Rendering Pipeline

SSR sends fully rendered HTML from server, CSR sends minimal HTML and renders on client, prerender sends static HTML built ahead. SSR and prerender reduce browser layout and paint delays by providing ready content.

→HTML Parsing

→Style Calculation

→Layout

→Paint

→Composite

⚠️ BottleneckLayout and Paint stages are delayed in CSR due to missing initial content.

Core Web Vital Affected

LCP, INP

This affects how fast the main content appears and how quickly users can interact with the page.

Optimization Tips

1Use SSR to deliver ready HTML for dynamic pages to improve LCP and INP.

2Use prerender for mostly static pages to serve fast-loading static HTML.

3Avoid disabling SSR unless you need full client control, as it delays content display.

Performance Quiz - 3 Questions

Which page option usually gives the fastest initial content display?

A Rendering only on user interaction

B Client-Side Rendering (CSR)

C Server-Side Rendering (SSR)

D No rendering at all

DevTools: Performance

How to check: Record page load, look for 'First Contentful Paint' and 'Time to Interactive' timings.

What to look for: Shorter LCP and INP times indicate good SSR or prerender usage; long delays suggest CSR bottlenecks.

Practice

1. What does SSR (Server-Side Rendering) do in a Svelte page?

easy

A. It sends fully rendered HTML from the server to the browser.

B. It renders the page only after JavaScript loads in the browser.

C. It builds static pages ahead of time during the build process.

D. It disables JavaScript on the page for faster loading.

Solution

1. Step 1: Understand SSR concept

   SSR means the server prepares the full HTML before sending it to the browser.

2. Step 2: Compare with other options

   CSR waits for JavaScript to render, prerender builds static pages ahead, and disabling JavaScript is unrelated.

3. Final Answer:

   It sends fully rendered HTML from the server to the browser. -> Option A

4. Quick Check:

   SSR = server sends ready HTML [OK]

Hint: SSR means server sends ready HTML, not browser rendering [OK]

Common Mistakes:

• Confusing SSR with CSR
• Thinking prerender is the same as SSR
• Believing SSR disables JavaScript

2. Which of the following is the correct way to disable SSR for a Svelte page?

easy

A. export const csr = true;

B. export const prerender = false;

C. export const ssr = false;

D. export const ssr = true;

Solution

1. Step 1: Recall syntax to disable SSR

   In Svelte, setting export const ssr = false; disables server-side rendering.

2. Step 2: Check other options

   prerender controls static generation, csr is not a valid option, and ssr = true enables SSR.

3. Final Answer:

   export const ssr = false; -> Option C

4. Quick Check:

   Disable SSR with ssr = false [OK]

Hint: Use 'export const ssr = false;' to disable SSR [OK]

Common Mistakes:

• Using 'prerender = false' to disable SSR
• Trying to set 'csr = true' which is invalid
• Confusing enabling and disabling SSR

3. Given this Svelte page code:

export const prerender = true;
export const ssr = false;

What will happen when this page is built and served?

medium

A. The page is statically generated at build time and rendered only on the client.

B. The page is server-rendered on each request and not prerendered.

C. The page is neither prerendered nor server-rendered, causing an error.

D. The page is prerendered and server-rendered on every request.

Solution

1. Step 1: Analyze prerender and ssr flags

   Setting prerender = true means the page is built as a static HTML file ahead of time. Setting ssr = false disables server-side rendering on requests.

2. Step 2: Understand combined effect

   The page is generated once at build time (prerendered) and then served as static HTML. Since SSR is off, no server rendering happens on requests; rendering happens on the client.

3. Final Answer:

   The page is statically generated at build time and rendered only on the client. -> Option A

4. Quick Check:

   prerender true + ssr false = static build + client render [OK]

Hint: prerender true + ssr false means static build + client render [OK]

Common Mistakes:

• Thinking SSR still runs on requests
• Assuming no rendering happens causing error
• Confusing prerender with SSR

4. You want a Svelte page to be fully static and load instantly without server rendering. You write:

export const ssr = false;
export const prerender = false;

What is the problem with this setup?

medium

A. The page will be server-rendered but not prerendered, causing slow loads.

B. The page will not prerender and also disables SSR, so it won't generate HTML before client loads.

C. The page will cause a syntax error because both options cannot be false.

D. The page will prerender but still use SSR, causing duplicate rendering.

Solution

1. Step 1: Understand the effect of both flags false

   Setting ssr = false disables server-side rendering, and prerender = false disables static generation.

2. Step 2: Identify the consequence

   Without SSR or prerender, the page has no prebuilt HTML and relies on client-side rendering only, causing a blank page until JavaScript runs.

3. Final Answer:

   The page will not prerender and also disables SSR, so it won't generate HTML before client loads. -> Option B

4. Quick Check:

   ssr false + prerender false = no prebuilt HTML [OK]

Hint: Both ssr and prerender false means no prebuilt HTML [OK]

Common Mistakes:

• Assuming prerender happens automatically
• Thinking this causes syntax error
• Believing SSR still runs when false

5. You want a Svelte page that:
- Is prerendered as static HTML for fast loading
- Uses SSR only if the user is logged in
- Otherwise, renders on the client

Which setup best achieves this behavior?

hard

A. export const prerender = false;
export const ssr = false;
// Fully client-side rendering only

B. export const prerender = false;
export const ssr = true;
// Server renders always regardless of login

C. export const prerender = true;
export const ssr = false;
// Use client-side code to check login and fetch data

D. export const prerender = true;
export const ssr = true;
// Use server hooks to check login and render accordingly

Solution

1. Step 1: Understand prerender and SSR roles

   Prerender true builds static HTML at build time. SSR true enables server rendering on requests.

2. Step 2: Match requirements

   To prerender for fast loading and also use SSR conditionally (like checking login), you need both prerender and SSR enabled. Server hooks can check login and decide rendering.

3. Step 3: Evaluate other options

   export const prerender = true; export const ssr = false; // Use client-side code to check login and fetch data disables SSR, so no server rendering on login. export const prerender = false; export const ssr = true; // Server renders always regardless of login disables prerender, losing static HTML. export const prerender = false; export const ssr = false; // Fully client-side rendering only disables both, no server or static rendering.

4. Final Answer:

   export const prerender = true; export const ssr = true; // Use server hooks to check login and render accordingly -> Option D

5. Quick Check:

   Prerender + SSR true for static + conditional server render [OK]

Hint: Enable both prerender and SSR for static plus conditional server render [OK]

Common Mistakes:

• Disabling SSR when conditional server render is needed
• Disabling prerender losing static HTML benefits
• Assuming client code can replace server login checks