WordPress Speed Optimization: A Developer-Led Core Web Vitals Checklist

If your WordPress site feels slow, you do not just lose patience. You lose leads, rankings, and revenue. Google measures real user experience with Core Web Vitals, and the same issues that hurt those metrics usually hurt conversion rates too.

Below is a developer-led checklist I use to improve performance without breaking layouts, editors, or tracking. It is not about installing one more plugin. It is about fixing the root causes.

What Core Web Vitals actually measure

LCP (Largest Contentful Paint) measures how quickly the main content becomes visible.

CLS (Cumulative Layout Shift) measures visual stability, meaning whether the page jumps while loading.

INP (Interaction to Next Paint) measures responsiveness when users click, tap, or type.

Step 1: Run the right diagnostics (and capture a baseline)

Before changing anything, capture a baseline so you can prove improvement and avoid guesswork. I typically check:

• PageSpeed Insights (mobile and desktop) for lab signals and field data when available
• Chrome DevTools Lighthouse and Performance panel for waterfalls and long tasks
• WebPageTest for advanced waterfalls and repeat view behavior
• Server response time (TTFB) and caching headers

Tip: Save screenshots of the metrics and the waterfall before changes. It makes client reporting and troubleshooting much easier.

Step 2: Fix the biggest LCP killers

Most slow WordPress pages have one or more of these LCP issues:

• Unoptimized hero images (wrong format, wrong dimensions, too heavy)
• Render-blocking CSS and JavaScript
• Too many third-party scripts (chat widgets, analytics stacks, heatmaps)
• Slow backend or uncached pages

Image optimization (the non-negotiables)

• Convert large images to WebP (or AVIF where supported and safe for your setup)
• Serve correctly sized images, not a 4000px file in a 1200px container
• Compress without visible quality loss
• Preload the hero image when it is the LCP element
• Lazy-load below-the-fold images only (do not lazy-load the hero)

CSS and JavaScript: reduce render blocking

To improve LCP and INP, the goal is to reduce work on the main thread and delay non-critical resources. Common actions include:

• Generate critical CSS for above-the-fold content
• Defer non-critical scripts
• Delay third-party scripts until interaction when appropriate
• Remove unused CSS and JS (page builders often add a lot of it)

Step 3: Eliminate CLS (layout shifts)

CLS is often caused by missing size attributes, late-loading fonts, or content injected by scripts. I fix it by:

• Setting explicit width and height for images and embeds
• Reserving space for banners, cookie notices, and popups
• Using font-display properly and preloading critical fonts
• Auditing sliders and sticky headers that change height on load

Step 4: Improve server-side performance (TTFB matters)

If the server is slow, front-end tweaks will hit a ceiling. Typical improvements include:

• Full-page caching for public pages
• Object caching (Redis or similar) when the site benefits from it
• Database cleanup and reducing autoloaded options
• Upgrading PHP versions and removing slow legacy plugins
• Using a CDN for static assets

Step 5: Keep SEO and tracking intact

Performance work can accidentally break SEO and analytics if it is done carelessly. During optimization I always verify:

• Robots.txt and sitemap.xml are correct and accessible
• Canonical tags and indexing directives did not change
• Important scripts (analytics, pixels) still fire correctly
• Redirects work as expected and do not create chains

When to involve a developer (and when a plugin is enough)

If your issues are mainly image weight and basic caching, a plugin configuration may be enough. If you have poor INP, heavy page builder output, or conflicts between optimization plugins and editors, you usually need developer-led changes to avoid breaking the site.

Need help improving your WordPress performance?

If you want a clean audit, a prioritized fix plan, and measurable Core Web Vitals improvements, we can help. Contact us and share your PageSpeed link and the page that matters most for conversions.

Contact iDev to get started.