Why your site feels slow on a phone

A site can feel instant on your office computer and sluggish on a customer's phone. Phones have slower processors and often slower connections, so whatever is heavy shows up there first.

Images, almost always

The biggest cause is usually pictures uploaded straight from a camera, thousands of pixels wide, shown in a space a few hundred pixels wide. Serve each image at the size it's shown, in a modern format like WebP or AVIF, and let images further down the page load only when they're needed:

<img
  src="/photos/shop-800.webp"
  width="800" height="600"
  loading="lazy" decoding="async"
  alt="The shop from the street"
/>

The exception is the main picture at the top. Don't lazy-load that one; tell the browser it matters most instead.

Fonts

Every font family and weight is another download. Two weights of one family is usually plenty. Use font-display: swap so text shows straight away in a fallback font instead of staying invisible.

Other people's scripts

Chat widgets, trackers, embedded maps and video players each bring their own code, often more than the site itself. Keep only the ones you use, and load the rest after the page is ready or when someone taps them.

Measure on a phone, not a laptop

Google's PageSpeed Insights tests your page as a mid-range phone on a slow connection. It reports three Core Web Vitals, with these thresholds for 'good':

• Largest Contentful Paint, how soon the main content shows: under 2.5 seconds.
• Interaction to Next Paint, how quickly the page responds to a tap: under 200 milliseconds.
• Cumulative Layout Shift, how much things jump around while loading: under 0.1.

“Speed isn't one big fix. It's not sending what the visitor doesn't need yet.”