Why SSG/SSR Beat Other Approaches for Image Loading (and Core Web Vitals)

Quick definitions (30 seconds)

SSG (Static Site Generation): Pages are pre-rendered at build time to HTML and deployed on a CDN. (e.g., getStaticProps in Next.js.)

SSR (Server-Side Rendering): Pages are rendered to HTML on the server per request—still cacheable at the CDN edge. (e.g., getServerSideProps in Next.js.)

CSR (Client-Side Rendering): Browser downloads a JS bundle and builds the page on the client before content appears. Common in SPA setups.

Why SSG/SSR deliver images better than CSR

1) Faster LCP (Largest Contentful Paint)

With SSG/SSR, the hero image and critical markup are ready in the first HTML. The browser can request the image immediately, rather than waiting for a JS bundle to bootstrap the page (CSR). That alone can shave hundreds of milliseconds off LCP.

What it means: Users see the main visual sooner → lower bounce, better rankings.

2) Earlier discovery & preloading

Because the server sends proper HTML, the browser discovers the hero <img> (or <Image> placeholder) during initial parse. You can also add <link rel="preload" as="image"> for the LCP asset. CSR often delays this discovery until after hydration.

Result: Your heaviest image is fetched earlier, not after JS executes.

3) Automatic responsive images & format negotiation

Modern SSG/SSR stacks (e.g., Next.js next/image) generate responsive srcsets and serve WebP/AVIF when supported—on the server. CSR setups frequently leave this on the developer or rely on client-side libraries that still wait on JS before choosing sources.

Effect: Less over-fetching on mobile, smaller files, faster paints.

4) Edge caching + immutable delivery

SSG pages are static and distributed to a CDN edge. SSR can also be cached at the edge with short TTLs or revalidation. Images live behind a CDN image loader (e.g., Next.js Image Optimization or a dedicated Image CDN), so they're transformed once and served from the closest PoP.

Why it matters: Low latency + consistent performance worldwide.

5) Fewer layout shifts (better CLS)

Server rendering lets you set intrinsic dimensions (width/height or aspect-ratio) in HTML, and image components reserve space before the file arrives. CSR often "discovers" dimensions late (after JS runs), causing layout jumps.

Outcome: Stable layout → better CLS and higher perceived quality.

6) SEO & crawl efficiency

Search engines get the content and images in HTML immediately (no or minimal JS required to see primary content). That makes discovery and indexing simpler, particularly for image search and rich result eligibility.

Bottom line: Better crawl > more impressions > more clicks. Learn more about our SEO services.

Practical setup (Next.js example)

Choose SSG first, SSR when you must

• Marketing, blogs, brochures: SSG (generateStaticParams / getStaticProps + revalidate for ISR).
• Rapidly changing data or per-user content: SSR (getServerSideProps) with HTTP caching or edge middleware.

Use the <Image /> component (Next.js)

It outputs responsive <img> with srcset, lazy-loading, decoding, and modern formats.

Always specify width and height or fill with a parent aspect-ratio to prevent CLS.

Mark the LCP image with priority in Next 13/14 (app router: <Image priority … />).

import Image from "next/image";

export default function Hero() {
  return (
    <div className="hero">
      <Image
        src="/images/hero.avif"
        alt="Your brand's hero visual"
        width={1600}
        height={900}
        priority // preloads for LCP
        sizes="(max-width: 768px) 100vw, 1600px"
      />
      <h1>Fast, SEO-Ready Websites</h1>
    </div>
  );
}

Preload the actual LCP asset

In the Head for the LCP route, add:

<link rel="preload" as="image" href="/images/hero.avif" 
  imagesrcset="/images/hero.avif 1600w, /images/hero-800.avif 800w" 
  imagesizes="(max-width:768px) 100vw, 1600px">

(Next.js handles much of this when priority is set, but explicit preloads give you control.)

Serve modern formats automatically

• Store a high-quality source (PNG/JPG). Let the image optimizer output WebP/AVIF based on the request.
• Set long-lived cache headers for transformed variants.

Lazy-load everything below the fold

• The <Image /> component defaults to loading="lazy" for non-priority images.
• Defer non-critical images (carousels, galleries, reviews) to keep initial bytes low.

Keep fonts from blocking images

• Self-host fonts, subset, and add font-display: swap.
• Preload critical font files only if they're actually used above the fold.

SSG/SSR vs other image strategies (at a glance)

Measurable wins you'll see

• LCP down (often by 300–900 ms) because the hero image starts sooner.
• CLS down from reserving space server-side.
• INP steadier because less JS runs before first interaction.
• Better organic performance as CWV improves and HTML is index-ready.
• Lower CDN egress from responsive srcsets and cache hits.

10-point checklist (copy/paste)

1. Prefer SSG; use ISR (revalidate) to keep content fresh.
2. Use SSR only where personalization or fast-changing data requires it.
3. Render the LCP image in HTML; mark it priority (Next.js).
4. Reserve dimensions (width/height or aspect-ratio) for every image.
5. Serve WebP/AVIF; keep hero ≤ ~250–400 KB.
6. Add sizes/srcset so mobile doesn't fetch desktop files.
7. Lazy-load below-the-fold media.
8. Preload the true LCP asset (and critical font if necessary).
9. Cache pages and image variants at the edge (immutable headers).
10. Re-measure CWV (PageSpeed, GSC) and adjust per route.

FAQ

Is SSG always better than SSR?

For marketing and editorial pages, yes—SSG with ISR gives CDN-level speed. Use SSR when data changes per request or must be fresh every time (and cache where possible).

Do I still need an image CDN if I use <Image />?

Next.js includes one. Dedicated image CDNs add advanced transforms, but many sites perform great with the built-in optimizer + global hosting.

What about SPAs?

You can ship fast SPAs, but you must solve early HTML, preloading, and dimension reservation manually. SSG/SSR remove most foot-guns by design. Learn why custom code beats templates for performance and SEO.

Bottom line

If you want faster loads, better SEO, and cleaner UX, move critical pages to SSG/SSR and let the server handle image discovery, optimization, and caching. Pair that with responsive images, modern formats, and proper sizing—and you'll see Core Web Vitals and conversions rise without heroics.