Why your website is slow on phones, and how to fix it

01The three numbers that matter

Google's Core Web Vitals measure three things a visitor feels:

• Largest Contentful Paint (LCP): how fast the main content appears. Good is within 2.5 seconds.
• Interaction to Next Paint (INP): how fast the page responds when someone taps or types. Good is 200 milliseconds or less.
• Cumulative Layout Shift (CLS): how much the page jumps around while loading. Good is 0.1 or less.

Google measures these at the 75th percentile of page loads, separately for mobile and desktop, so a page passes only when most real visits are good. ¹ INP replaced the older First Input Delay metric in March 2024. ²

02Does speed affect Google rankings?

Yes, but it is not magic. Google confirms that Core Web Vitals are used by our ranking systems, and also that good scores do not guarantee top positions, because relevance still comes first. ³ Speed is the tie-breaker between similar pages, and the reason visitors stay long enough to enquire.

03Why phones suffer most

A page that feels instant on an office laptop can crawl on a mid-range phone on a train. Phones have less processing power to run JavaScript, and mobile connections vary. The usual culprits:

• A huge hero image or video that is not resized for phones
• Sliders, chat widgets and tracking scripts loading before the content
• Web fonts that block text from appearing
• Ads, banners or images without reserved space that push the page around

04Five fixes, in order of impact

1. Fix the largest image. Resize it per screen size, serve it as WebP or AVIF and load it first. Lazy-load everything below the fold.
2. Cut JavaScript. Remove plugins you don't use, delay chat and tracking until after the page is usable, and break long tasks into smaller ones.
3. Reserve space. Give every image, video and embed a width and height so nothing jumps when it loads.
4. Load fonts politely. Use fewer font files, preload the main one and let text show straight away.
5. Cache and compress. Serve files from a CDN with long cache times and modern compression.

05How to measure your own site

Use Google Search Console's Core Web Vitals report for real-user data across your site, and PageSpeed Insights to test one page at a time. Lab scores are useful for debugging; field data from real visitors is what counts.

This website measures its own Core Web Vitals live in your browser on the home page and About page. Nothing there is hard-coded.

FAQQuestions people ask

SRCSources

1. Web Vitalsweb.dev (Google)
2. Interaction to Next Paint becomes a Core Web Vital on March 12web.dev (Google)
3. Understanding page experience in Google Search resultsGoogle Search Central

Every figure links to its original source. Checked on 18 October 2026.