Website speed optimisation is the process of systematically improving webpage load and interaction times to boost user engagement and search ranking. The industry standard framework for measuring this is Google’s Core Web Vitals, which tracks three primary signals: Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS). A well-structured website speed optimization checklist addresses each of these signals in priority order, starting with the fixes that deliver the greatest measurable gains. MedwayWebDesign builds every client site with these performance standards as a foundation, not an afterthought. Working through this checklist systematically is the fastest route from a slow, underperforming site to one that ranks, converts, and retains visitors.
1. How to assess current website speed performance
Measurement comes before any fix. Without a baseline, you cannot know which changes actually move the needle on your site’s performance checklist.
The primary tools for this work are Google PageSpeed Insights, Google Lighthouse, and Google Search Console. PageSpeed Insights combines both lab data and real-user field data in a single report. Lighthouse runs a synthetic audit directly in Chrome DevTools and scores your page across performance, accessibility, and best practices. Search Console surfaces Core Web Vitals data aggregated from real visitors, segmented by mobile and desktop.
Field data reflects genuine bottlenecks more accurately than synthetic metrics alone. Google measures Core Web Vitals at the 75th percentile of real user experiences, meaning your score must be good for the vast majority of visitors, not just the average. Lab tools simulate a single controlled session. Field data shows what your actual audience encounters on their devices and networks.
When auditing, prioritise your highest-traffic pages first. A homepage, a key landing page, and your top product or service page cover the most business-critical ground. For each page, note the LCP element, the INP score, and the CLS value. Then inventory every script loaded on the page and flag any large image or video assets.
Pro Tip: Run your audit on a mobile connection simulation, not just desktop. Google’s ranking signals use mobile-first indexing, so mobile field data carries the most weight for search performance.
2. Eliminate render-blocking JavaScript
Render-blocking JavaScript is the single most common cause of slow LCP scores. When a browser encounters a script tag without async or defer attributes, it stops parsing the HTML entirely until that script downloads and executes. Every millisecond spent waiting on a script is a millisecond the user sees nothing.
JavaScript is the most expensive resource for browsers because of parsing and execution work, not just download size. A 100KB JavaScript file costs far more in processing time than a 100KB image. Adding defer to non-critical scripts and async to independent scripts removes the render block without breaking functionality.
Beyond attributes, reduce the JavaScript bundle size by removing unused scripts entirely. Audit your installed plugins and third-party tags. Many sites carry scripts from abandoned A/B tests, old analytics integrations, or marketing tools that no longer run. Each one adds to the main-thread workload and delays INP.
Pro Tip: Use Chrome DevTools Coverage tab to identify exactly which JavaScript and CSS bytes are unused on each page load. This gives you a precise removal list rather than guesswork.
3. Compress and convert images to modern formats
Images are the LCP element on 70–80% of pages. That makes image optimisation the highest-return action on any site speed optimisation checklist. Converting images to WebP or AVIF formats reduces image payload by 40–60% without visible quality loss. That reduction directly shortens the time the browser spends downloading the LCP element.
Compression alone is not sufficient. You must also serve correctly sized images. Sending a 2,400px wide image to a mobile screen that renders it at 400px wastes bandwidth on every mobile visit. Use the HTML srcset attribute to serve different image sizes to different viewports. This is a core part of mobile ecommerce performance that many site owners overlook.
Lazy loading below-the-fold images with the loading="lazy" attribute reduces initial page weight. The browser only fetches those images when the visitor scrolls toward them. Never apply lazy loading to your LCP image, as that delays the most important visual element.
4. Fix layout shifts with explicit dimensions
Cumulative Layout Shift measures visual instability. A page that jumps as it loads frustrates visitors and signals poor quality to Google. Setting explicit width and height attributes on every image and embed prevents the browser from reserving zero space for an element and then reflowing the page when the asset arrives.
Layout shifts most commonly come from images without dimensions, web fonts swapping in late, and banner or widget injections from third-party scripts. Each of these has a specific fix. Images need width and height attributes in the HTML. Fonts need the font-display: swap property in CSS. Third-party banners need reserved space via CSS aspect-ratio or fixed-height containers.
CLS is the Core Web Vitals metric most directly tied to responsive design principles. A layout that shifts on mobile but not desktop will still fail the field data threshold, because Google scores at the 75th percentile across all sessions.
5. Implement a caching strategy
Caching reduces the number of full server round-trips a returning visitor triggers. Browser caching stores static assets locally on the visitor’s device. Page caching serves pre-built HTML files instead of regenerating pages on every request. A Content Delivery Network (CDN) caches assets on servers geographically close to each visitor.
Using a single well-maintained caching plugin is the correct approach for WordPress sites. Stacking multiple caching plugins causes conflicts and obscures the real bottlenecks. Two plugins may each attempt to manage the same cache files, producing corrupted output or preventing cache from building at all. Choose one plugin, configure it correctly, and test after every configuration change.
CDN caching delivers the most consistent gains for sites with international or geographically spread audiences. Static assets served from a CDN edge node close to the visitor load faster than assets served from a single origin server. For product page optimisation, CDN caching is particularly valuable because product images are typically large and frequently requested.
6. Optimise web font delivery
Web fonts are a common source of both render-blocking behaviour and layout shift. The font-display: swap CSS property instructs the browser to display text in a fallback font immediately, then swap to the web font once it loads. This prevents invisible text during load, which harms both user experience and LCP.
Reduce the number of font weights and styles loaded. Each font weight is a separate file download. A site loading six font weights across two typefaces may be downloading twelve font files per page. Audit which weights actually appear in your design and remove the rest. Subsetting fonts to include only the character sets you use further reduces file size.
Self-hosting fonts removes a DNS lookup to a third-party font service. Every external domain a page must connect to adds latency. Hosting font files on your own server or CDN eliminates that dependency entirely.
7. Remove unused CSS and manage third-party scripts
Unused CSS adds to the stylesheet download size and increases the browser’s style calculation work. Tools like Chrome DevTools Coverage and PurgeCSS identify CSS rules that are never applied on a given page. Removing them reduces both payload and rendering time.
Third-party scripts, including live chat widgets, social media embeds, and advertising tags, are among the largest contributors to slow INP scores. Each script executes on the main thread, competing with the browser’s response to user interactions. For critical pages such as checkout or lead generation forms, defer or conditionally load third-party scripts so they do not block the primary user journey.
Performance budgets with defined maximum sizes for JavaScript, CSS, and total page weight prevent regression after launch. Integrating budget checks into a CI/CD pipeline means any code change that breaches the budget fails the build automatically. This enforces speed discipline across the entire development team, not just during an initial audit.
8. Avoid common pitfalls in site speed work
The most damaging mistake in site speed work is fixing things before measuring them. Premature optimisation wastes development time on changes that do not move the metrics that matter. Always identify your top three bottlenecks from field data before writing a single line of code.
A second common error is treating speed work as a one-time project. Treating speed as an ongoing publishing rule rather than a one-time project significantly improves performance and avoids regressions. Every time a new image is uploaded, a new plugin is installed, or a new marketing script is added, the performance baseline can shift. Build speed checks into your content publishing workflow.
“Order of operations matters more than checklist length for meaningful speed gains.” This principle, drawn from expert performance analysis, means that fixing render-blocking JavaScript before addressing font loading will always yield faster results than working through a checklist alphabetically.
A third pitfall is over-relying on synthetic scores. A PageSpeed Insights score of 90 in a lab simulation does not guarantee a good experience for real users on slower devices. Cross-reference lab scores with field data from Search Console before declaring a page fully optimised.
Key takeaways
A site’s performance is determined by the order in which fixes are applied, not the total number of changes made.
| Point | Details |
|---|---|
| Measure before fixing | Use Google PageSpeed Insights and Search Console field data to identify real bottlenecks first. |
| Images drive LCP | Converting to WebP or AVIF and serving correctly sized images delivers the largest single speed gain. |
| One caching plugin only | Stacking multiple caching plugins causes conflicts; choose one and configure it correctly. |
| Speed is a publishing habit | Build image compression and script review into every content update, not just launch audits. |
| Performance budgets prevent regression | Define maximum file size limits for JavaScript and CSS and enforce them in your deployment process. |
Why prioritisation is the real skill in speed optimisation
Most website owners approach speed work as a checklist to complete from top to bottom. In my experience, that approach produces modest gains at best. The sites I have seen achieve the most dramatic improvements are those where the owner or developer identified the single largest bottleneck first and fixed only that before measuring again.
The reason this matters is compounding. Fixing render-blocking JavaScript often reveals that the LCP image was already loading quickly. Fixing the LCP image often reveals that CLS was the remaining problem. Each fix changes the performance profile of the page, and the next priority shifts accordingly. Working through a fixed list ignores this dynamic entirely.
The maintenance mindset is equally underrated. Speed work done at launch degrades within months if there is no publishing discipline. I recommend building a short checklist into your content management workflow: compress every image before upload, check for new third-party scripts monthly, and run a Lighthouse audit after any significant plugin or theme update. This takes less than ten minutes per publishing cycle and prevents the slow accumulation of performance debt that forces a full audit every year.
The role of web design in business growth is inseparable from speed. A visually strong site that loads slowly loses visitors before they see the design. Speed and design are not competing priorities. They are the same priority.
— Ian Rickard
How MedwayWebDesign supports faster, higher-performing websites
Speed-conscious design is built into every project MedwayWebDesign delivers. Rather than applying performance fixes retrospectively, the agency structures each build around Core Web Vitals targets from the initial architecture stage, reducing the remediation work that typically follows a standard launch.
For small business owners who want a site built to perform from day one, MedwayWebDesign’s custom web design service covers image optimisation, caching configuration, and script management as standard deliverables. The result is a site that passes field data thresholds, not just lab simulations. If your current site is failing Core Web Vitals or losing visitors to slow load times, that is a problem with a defined solution.
FAQ
What are Core Web Vitals?
Core Web Vitals are Google’s three primary user experience metrics: Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS). Google measures each at the 75th percentile of real user sessions.
How do I check my website’s speed for free?
Google PageSpeed Insights and Google Lighthouse are both free tools that provide lab scores and field data. Search Console also surfaces Core Web Vitals data segmented by page type and device.
What is the fastest way to improve loading speed?
Converting images to WebP or AVIF format and eliminating render-blocking JavaScript deliver the largest gains in the shortest time. These two fixes address the LCP and main-thread bottlenecks that affect most sites.
How often should I run a website performance checklist?
Run a full audit after any significant site update, plugin change, or new third-party script addition. A lighter check covering image compression and Lighthouse scores should be part of every content publishing cycle.
Does site speed affect SEO rankings?
Yes. Google uses Core Web Vitals as a ranking signal, and field data from real users directly influences how pages are assessed. A site that fails LCP or CLS thresholds is at a measurable disadvantage in competitive search results.