Written by IILL Editorial. Record the fixture, device, network, and release version when comparing results.
“The mobile site feels slow” is not yet a debugging plan. Open the same URL under the same device and network conditions, then record three different questions: when the main content appears (LCP), when an interaction responds (INP), and whether the layout moves while reading (CLS). These metrics help you find problems; they do not guarantee rankings or more enquiries.
Read the three metrics as different problems
| Metric | Question | First places to inspect |
|---|---|---|
| LCP | When does the main above-the-fold content appear? | Hero image, server response, blocking CSS and fonts |
| INP | How quickly do menus, buttons, and forms respond? | Long JavaScript tasks, event handlers, third-party scripts |
| CLS | Does the page move while someone is reading? | Images without dimensions, late banners, font swaps |
web.dev’s Core Web Vitals overview explains the different user experiences represented by each metric. A lower LCP is not enough if a mobile contact form still fails or its success state is unclear.
Fix the measurement conditions first
Record these values before comparing one deployment with another:
- final URL and redirect behavior;
- device, browser, and viewport;
- network and cache state and the test time;
- deployment commit or asset version;
- login, personalization, ads, and third-party widgets.
Do not present one Lighthouse run as a customer benchmark. Repeat the check under the same conditions and record which change preceded the new observation.
A mobile-first pre-launch sequence
1. Find the largest first-screen element
Check whether a hero image is oversized or an off-screen carousel is loaded before the content that people came to read. Give images a display size and suitable format, and defer media below the first screen when it is safe. Use the LCP optimisation guide for causes and fixes, then verify the actual HTML and response of the site.
2. Use the page with one finger
Open the menu, expand an FAQ, move through the enquiry form, and trigger a validation error. An event being logged is not the same as a screen responding quickly. If INP is poor, narrow the cause by reducing long JavaScript tasks and deferring non-essential analytics, chat, or animation.
3. Find every layout shift
Reserve space for images, video, and banners. Check that a font swap does not push a heading or CTA below the fold. Do not hide important content just to improve a score; stabilise its space and order instead.
4. Test speed and conversion together
A fast page still fails its job when the enquiry button does not submit. On mobile, run the CTA, form start, invalid submission, success response, and failure message once each. Keep the server-confirmed success event as the primary conversion and link the measurement contract to IILL’s landing-page GA4 guide.
What to check after launch
On day one, verify the canonical URL, mobile menu, form delivery, 404s, and redirects. During the first week, check the deployed assets and external scripts, plus indexing and Core Web Vitals warnings in Search Console. After 28 days, compare search landings with valid GA4 enquiry completions. Do not infer a ranking or revenue cause from one metric; ship one coherent change, record its date, and wait for a comparable window.
Pre-launch checklist
- URL, device, viewport, network, and release version are recorded.
- LCP, INP, and CLS are compared under the same conditions.
- Hero images and fonts do not block the first useful content.
- Menus, forms, errors, and success states work on a narrow screen.
- Images, banners, and fonts do not move the layout unexpectedly.
- CTA and enquiry-success events are collected once, without personal form data.
- Canonical, sitemap, robots, title, and description match the rendered page.
- No single score is presented as a ranking or conversion guarantee.
See IILL’s website production timeline for launch dependencies and the maintenance-scope guide for post-launch ownership. IILL’s project contact page connects performance checks with content, forms, search, analytics, and handover responsibility.
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