Website Speed Optimization & Core Web Vitals

Website speed directly impacts your bottom line. Studies show that a one-second delay in page load time can reduce conversions by seven percent, and Google uses page speed as a ranking factor. Website speed optimization at Kosmoweb is a systematic process that identifies and eliminates performance bottlenecks, delivering faster load times that improve both user experience and search rankings.

Our speed optimization process includes image compression and next-gen format conversion, code minification and bundling, server response time improvements, browser caching configuration, lazy loading implementation, and critical rendering path optimization. We measure results using Google PageSpeed Insights, Core Web Vitals, and real-user monitoring data to ensure improvements translate into better experiences for your actual visitors.

Performance is not a one-time fix. As you add content, install plugins, or update your platform, site speed can degrade over time. Our website management services include regular performance audits and proactive optimization to keep your site fast. Whether your site is built on WordPress, Shopify, or a custom platform using React and Next.js, we know how to squeeze every millisecond of performance.

Fast websites rank higher, convert better, and provide a superior user experience. Businesses in Prague and across the Czech Republic trust us to optimize their sites for speed. Combined with SEO optimization and thoughtful web design, speed optimization completes the trifecta of a high-performing website that delivers real business results.

Core Web Vitals in 2026 means three numbers: LCP under 2.5s (largest element painted), INP under 200ms (interaction response, replaced FID in March 2024), and CLS under 0.1 (cumulative layout shift). INP is the one most sites fail — it measures the worst delay across all user interactions, not just the first. Common INP killers: heavy JavaScript on click handlers, third-party tag managers running synchronously, oversized analytics bundles, and React hydration on slow devices. We measure with Lighthouse CI for synthetic data and the Chrome User Experience Report (CrUX) for real-world numbers from actual users, weighted toward the slowest 25%.

Images are still the biggest single payload on most websites. AVIF beats WebP by 20-50% at the same visual quality, WebP beats JPEG by 25-35%, and modern browsers support all three — so serve them via the picture element with srcset and proper sizes attributes. Native lazy loading (loading='lazy') handles below-the-fold images cheaply; above-the-fold hero images need fetchpriority='high' to win the LCP race. On Next.js, next/image handles most of this automatically. On WordPress, Smush Pro, ShortPixel, or Imagify do the work — but only one of them, never stacked. We routinely cut total image weight 60-80% on first audit.

JavaScript is where most modern sites bleed performance. Route-level code splitting (free with Next.js App Router, opt-in with Vite/Webpack), tree-shaking unused exports, deferring non-critical third-party scripts (analytics, chat widgets, A/B tools), and replacing heavyweight libraries with smaller alternatives (date-fns over moment, day.js over moment, native fetch over axios) typically reclaim 200-500 KB per page. On React apps specifically, we audit for unnecessary client components — anything that does not need interactivity can be a Server Component and ship zero JS. The result usually visible in Lighthouse 'Reduce unused JavaScript' diagnostics, which most sites fail at 300+ KB.

CDN choice matters more for Czech sites than most agencies admit. Cloudflare's free tier covers basic edge caching, DDoS protection, and Brotli compression — fine for marketing sites. Bunny.net at €1-5/month per site has EU POPs in Prague and Vienna with sub-20ms latency to Czech users and image optimization for an extra fee, which is genuinely excellent value. Fastly is the enterprise option for media-heavy or commerce sites needing fine-grained cache control via VCL. Vercel Edge bundles CDN with Next.js hosting and handles ISR cache invalidation natively, which is a real productivity win. Skip Cloudflare Pro tier for most cases — it does not move the needle.

SSR, SSG, and ISR each fit different page types. Static generation (SSG) for marketing pages, blog posts, documentation — built once at deploy, served from CDN, sub-100ms TTFB. Incremental Static Regeneration (ISR) for content that changes daily or weekly without redeploy: product listings, category pages, news. Server-side rendering (SSR) for personalized routes: logged-in dashboards, cart, account. The trap most teams fall into is using SSR everywhere because it's the default, which means every request hits the server and TTFB lands at 600-1200ms instead of 50-100ms. We map each page type to the right strategy during the development phase.

WordPress speed optimization follows a predictable order: page caching (WP Rocket €60/year or LiteSpeed Cache free for LiteSpeed servers), image optimization via ShortPixel or Imagify, plugin audit and removal of anything contributing less than its weight in features, database cleanup (transients, post revisions, spam comments), CDN integration, and finally hosting tier upgrade if shared hosting is the bottleneck. Most WordPress sites we audit get from LCP 5s to LCP 2s through plugin removal and proper caching alone — the hosting upgrade is the last step, not the first. Throwing money at hosting before fixing the front-end is a classic mistake.

E-commerce speed has unique pressure points. Product images need responsive srcset to avoid serving 2000px hero shots to phones. Infinite scroll product listings need virtual scrolling or pagination — rendering 500 cards at once kills mobile devices. Filter performance is critical: client-side filtering of large catalogs locks the main thread; server-side filtering with debounced queries and skeleton loaders feels faster despite being slower in raw ms. Cart and checkout must use code-splitting aggressively — there is no reason to ship checkout JavaScript on the homepage. We routinely cut Shopify and WooCommerce product pages from 4s LCP to 1.5s through these targeted fixes.

Lighthouse scores are a starting point, not a target. The 100/100 fetish has destroyed more sites than it has helped, because the score is unweighted relative to actual user impact. We read Lighthouse reports backwards: ignore the score, start with Diagnostics and Opportunities, prioritize the top three by 'Estimated Savings' weighted against implementation effort. CLS and LCP issues outrank 'Eliminate render-blocking resources' on a site that already loads in 2s. We also cross-reference with field data from Google Search Console's Core Web Vitals report — synthetic Lighthouse runs on a perfect connection lie about real-world performance for the bottom quartile of users.