Why file compression improves website speed

Performance · 2025-02-18 · 6 min read

If your site loads slowly, the culprit is almost always one of three things: oversized images, unoptimised JavaScript, or chatty third-party scripts. Of those, oversized files are the easiest to fix and have the biggest impact.

The speed → SEO chain

1. Smaller files transfer faster.
2. Faster transfers improve LCP and FCP.
3. Better Core Web Vitals improve Google ranking signals.
4. Higher ranking = more clicks.
5. Faster pages = higher conversion (≈1% lift per 100ms in our reading of the data).

What to compress first

• Hero images: Usually the LCP element. Halving its weight cuts LCP nearly proportionally.
• Product imagery: E-commerce sites typically have hundreds of product photos at 2–4 MB each.
• Above-the-fold PDFs and brochures: Often linked directly from landing pages.
• Background videos: If you need a hero video, encode it efficiently (separate topic).

The 80/20 of compression

For 80% of websites, doing these three things produces 80%+ of the available speed gains:

1. Convert all photos to WebP at 82% quality.
2. Cap longest side at 1920px (desktop) or 1080px (mobile-first sites).
3. Compress any embedded PDFs with the PDF compressor.

Tools to use

For a one-time bulk pass over an existing image library:

• Shrink44 Bulk Compress — handles 1000+ files at once, no upload.
• Convert Image — PNG / JPG → WebP in one click.
• Resize Image — cap your longest-side dimensions.

What about Brotli / Gzip?

HTTP-level compression (Brotli / Gzip) helps with text-based files — HTML, CSS, JavaScript, SVG, JSON. It doesn't help with already-compressed binary files like JPG, PNG, WebP and PDF. You still need to compress images at the source.

Lighthouse score impact

In our testing, replacing a typical e-commerce product gallery's PNGs with WebP at 82% quality improved Lighthouse Performance scores by 18–32 points on mobile.