My site is slow on mobile but fine on desktop. Where do I start?

Phones have slower processors and often slower connections, so work that is invisible on a laptop becomes the bottleneck. Start with the right data, then fix the biggest problem first.

Measure with field data. Lab tests such as a single speed test are a snapshot. Field data comes from real visits. Search Console's Core Web Vitals report and PageSpeed Insights show field data when your site has enough traffic. Look at the mobile tab.

The 3 metrics, with the thresholds Google labels good:

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

Common causes of a mobile-only gap

1. A large hero image served at desktop size. Resize it and serve a smaller version on phones.
2. Too much JavaScript. Chat widgets, tag managers, A/B testing tools, and sliders all run on a weaker processor.
3. Fonts that block text from showing.
4. Banners, ads, and images without set dimensions that push content around.
5. Third-party scripts that load before the main content.

Order of work

1. Find the LCP element on your slowest template and fix that.
2. Remove or defer scripts you do not need.
3. Set width and height on images and reserve space for banners.
4. Retest in field data. It updates over about 28 days, so be patient.

A site audit that lists issues by template and by impact saves time here. Doily's audits include Core Web Vitals and rank the fixes.

The thresholds in one place.

A page passes Core Web Vitals when it meets all 3 at the 75th percentile of page loads, measured separately for mobile and desktop: LCP of 2.5 seconds or less, INP of 200 milliseconds or less, and CLS of 0.1 or less. INP replaced First Input Delay as a Core Web Vital in 2024.

A Lighthouse run on your own machine is a lab test, and it cannot measure INP at all. Use field data for the verdict and lab tests to find the cause.

Test on a real mid range phone.

If you can borrow one, load your slowest template on it over a normal connection. What you see is closer to what most visitors get than anything on a developer laptop, and the slow part is usually obvious within a few seconds: a late image, a script that freezes taps, or a banner that shoves the page down.