The verdict in three sentences
If your mid-sized company's website takes 4.8 seconds to render on mobile, you lose a measurable share of enquiries before buyers even read your value proposition. In London in 2026, a Core Web Vitals audit costs 1,500 to 4,000 GBP excl. VAT, and fixes 4,000 to 12,000 GBP depending on technical debt, over 3 to 6 weeks in total. Because each second saved adds 7 to 12 % in conversions on average, the return is measured in months, not years.
The three metrics and the thresholds to hit
Google assesses your visitors' real experience (CrUX field data, rolling 28 days) on three metrics. A B2B site heavy with images, tracking scripts and chat widgets often fails all three.
| Metric | What it measures | "Good" threshold | Typical value on an unoptimised B2B site |
|---|---|---|---|
| LCP (Largest Contentful Paint) | Render of the largest visible element | Under 2.5 s | 4 to 6 s |
| INP (Interaction to Next Paint) | Responsiveness to clicks and typing | Under 200 ms | 300 to 600 ms |
| CLS (Cumulative Layout Shift) | Visual stability while loading | Under 0.1 | 0.15 to 0.35 |
| TTFB (diagnostic metric) | Server response time | Under 0.8 s | 1.2 to 2 s |
| Homepage weight | Mobile download size | Under 1.5 MB | 4 to 8 MB |
| Third-party scripts | Tags, chat, maps, video | Fewer than 10 | 20 to 40 |
The usual culprits are well known: a 2 MB JPEG hero image, web fonts not preloaded, a tag manager firing 25 scripts at start-up, overloaded shared hosting and a WordPress theme built with a page builder.
What the audit and fixes cost
A proper audit goes well beyond a PageSpeed Insights screenshot. It combines field and lab data across 10 to 20 templates, then delivers a list prioritised by impact and cost.
| Service | London agency (GBP excl. VAT) | Independent consultant (GBP excl. VAT) | Lead time |
|---|---|---|---|
| Express audit (5 pages, summary report) | 1,500 to 2,200 | 900 to 1,400 | 3 to 5 days |
| Full audit (15 to 20 templates, prioritised plan) | 2,800 to 4,000 | 1,800 to 2,800 | 1 to 2 weeks |
| Image and font optimisation | 1,000 to 2,500 | 800 to 1,800 | 3 to 5 days |
| Third-party script and tag manager clean-up | 800 to 2,200 | 600 to 1,500 | 2 to 4 days |
| Front-end refactor (theme, JS, critical CSS) | 2,000 to 6,000 | 1,500 to 4,500 | 1 to 3 weeks |
| Hosting move and CDN | 400 to 1,200 + 30 to 150 per month | 400 to 900 | 2 to 3 days |
| Full fixes | 4,000 to 12,000 | 3,000 to 9,000 | 2 to 4 weeks |
Agencies cost more but cover development, hosting and follow-up; a consultant makes sense if your in-house team can apply the recommendations. Either way, demand a before and after measurement on field data, not just Lighthouse scores.
When the audit says you need a rebuild
If the audit prices more than 12,000 GBP of fixes on an ageing theme, compare it with a technical rebuild: above 60 % of the cost of rebuilding, patching the existing site is rarely the right call.
Mini case study
Emma, marketing manager at a 350-person London firm supplying industrial refrigeration equipment, sees a mobile LCP of 4.8 s. The site gets 9,000 visits per month and converts at 1.2 %, i.e. 108 enquiries, 10 % of which become opportunities worth 22,000 GBP on average, with a 25 % win rate.
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She orders a full audit at 3,200 GBP and fixes at 8,000 GBP. LCP drops to 2.1 s, a 2.7-second gain. Using a cautious assumption of +7 % conversion per second, the rate rises to about 1.43 %, i.e. 129 enquiries per month, 21 more. That means 2.1 extra opportunities and about 0.5 additional deal per month, or 11,500 GBP of extra monthly revenue. At a 25 % margin, the 11,200 GBP invested is recovered in about 4 months.
FAQ
How much does a Core Web Vitals audit cost in London in 2026?
An express audit costs 1,500 to 2,200 GBP and a full audit 2,800 to 4,000 GBP at an agency. An independent consultant charges roughly 35 % less, without handling the fixes.
What conversion gains can we expect?
Benchmark studies put the gain at 7 to 12 % in conversions per second of load time saved. On a B2B site going from 4.8 s to 2.5 s, a 15 to 25 % rise in enquiries is a realistic 2026 order of magnitude.
How long before Google reflects the change?
Fixes take 2 to 4 weeks, then CrUX data updates over a rolling 28 days. Allow 6 to 10 weeks from audit kick-off to full validation in Search Console.
Do we need to change hosting?
Only if TTFB regularly exceeds 0.8 s. Managed hosting with a CDN costs 30 to 150 GBP per month and can on its own save 0.5 to 1 second.
Do Core Web Vitals affect rankings?
Yes, they are part of the page experience signals, but they weigh less than content relevance. Their strongest effect is on converting the visitors you already have.
Let's scope your project. Send us your site URL and monthly visit and enquiry volumes: we estimate the expected gain, the audit plus fixes budget and a 3 to 6 week schedule. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.
