Speed requirements to put in a website contract

Key takeaways

If speed is not written into a website contract, it is not part of what you are buying. 'The site will be fast' cannot be tested, and a target score can be met on one page on one day. Useful speed requirements name the pages, the device, the measures and the pass marks, say how and when they will be checked, and state what happens if the site fails.

• Name page types, not just 'the website'.
• Specify mobile, with Core Web Vitals thresholds as the pass marks.
• Check at launch and again 28 days later, when real-visitor data exists.
• Agree what counts as the supplier's responsibility and what does not.

Why put speed in the contract?

Because design is approved on a large screen and speed is discovered later, on a phone, after the invoice is paid. Without a written requirement, a slow site is a disappointment, not a defect, and fixing it is extra work at extra cost.

A clear requirement also helps the supplier. It gives them grounds to refuse the video header or the sixth tracking tag that would break it.

• How website speed affects customer experience and conversions
• Questions to ask a web design agency before signing

Which targets should you specify?

Specify outcomes a third party could verify, using Google's published thresholds so nobody has to argue about the numbers.

• Largest Contentful Paint: 2.5 seconds or less on mobile.
• Interaction to Next Paint: 200 milliseconds or less.
• Cumulative Layout Shift: below 0.1.
• Server response: time to first byte within about 0.8 seconds.
• Forms: visible confirmation within one second of submission.
• Optional: first-load weight limit for key templates.

• Core Web Vitals for business websites
• Performance budgets: stop your website slowing down again

How should the test conditions be written?

State the conditions precisely, because the same page gives different results under different tests.

• 1. Pages: list each template, such as home, service, product, article, contact.
• 2. Device: mobile setting, with desktop as a secondary check.
• 3. Tool: name it, for example PageSpeed Insights.
• 4. Method: the median of three or five runs for lab results.
• 5. Timing: at launch for lab results, and after 28 days for field data.
• 6. Content: tested with real images and copy, not placeholders.

• Why do website speed tests give different results?
• Lab data vs field data: which speed numbers to believe

What is fair to hold a supplier to?

Hold a supplier to what they control: the theme or code, the templates, image handling, caching and the hosting they recommend. It is not fair to hold them to things you add or insist on.

• Supplier: templates, scripts they add, image sizing, caching, fonts.
• You: tracking tags, chat tools and widgets you request.
• You: images uploaded after launch at the wrong size.
• Shared: third-party tools both sides agreed to, recorded with their cost in speed.
• Either: hosting, depending on who chose it.

• Who should fix a slow website: developer, host or agency?
• Third-party scripts: the hidden reason your site is slow

What should happen if the site fails?

Say in advance. A reasonable clause gives the supplier a period to fix failures at their own cost where the cause is within their scope, and links final payment or sign-off to passing. It should also cover the months after launch: a short warranty during which speed regressions caused by the build are corrected.

Avoid demanding the impossible. A requirement for a lab score of 100, or for targets that ignore tools you have insisted on, invites either refusal or gaming.

• Website launch checklist

What does this look like in practice?

A pattern we see in disputes after launch: the brief said the site should be 'fast and modern'. The new site is slower than the old one on mobile. The supplier points to a desktop score. The client has nothing in writing to say that is not the test.

A single paragraph naming the templates, the mobile setting and the three thresholds would have settled it before work began. Global Bridge Labs (GBL) writes speed acceptance criteria into its own proposals for that reason.

• See client work
• Website Design & Development for UK businesses
• Mobile acceptance criteria to put in a website contract

Speed contract checklist

• List the page templates covered.
• State mobile thresholds for LCP, INP and CLS.
• Name the tool, method and number of runs.
• Set checks at launch and at 28 days.
• Record who is responsible for which scripts.
• Link sign-off to passing, with a fix period.

• A website brief template agencies can quote from

Next step

A paragraph in the contract is cheaper than an argument after launch. If you are briefing a new site, we can help you write requirements a supplier can price and you can test.