Core Web Vitals
for SPAs
Analyze Core Web Vitals issues specific to single-page applications. Detect CLS from hydration, LCP delays, and JavaScript blocking risks.
CLS Analysis
Detects images without dimensions and content injected by JavaScript that causes layout shifts.
LCP Optimization
Checks hero image preloading, fetchpriority hints, and above-the-fold lazy loading issues.
INP & Script Load
Analyzes inline JavaScript size, external script count, and render-blocking resources.
Analyze Core Web Vitals
Frequently Asked Questions
Why are Core Web Vitals different for SPAs?
Single-page applications face unique CWV challenges. Hydration can cause unexpected layout shifts (CLS), JavaScript bundles block the main thread (affecting INP), and hero images are often loaded late because they depend on JavaScript rendering instead of being in the initial HTML (hurting LCP).
What causes CLS in single-page applications?
CLS in SPAs is commonly caused by: images without width/height attributes, content injected by JavaScript during hydration that shifts the layout, dynamically loaded fonts, and lazy-loaded content above the fold. This tool checks for all these issues.
How can I improve LCP for my SPA?
Add preload hints for your hero image, use fetchpriority='high' on your most important image, never lazy-load above-the-fold images, and ensure your critical content is in the initial HTML rather than loaded by JavaScript. Prerendering can help by including rendered content in the initial response.
What is INP and how does JavaScript affect it?
INP (Interaction to Next Paint) measures responsiveness. Large JavaScript bundles block the main thread, making the page unresponsive to user input. Render-blocking scripts without defer/async, excessive inline JavaScript, and too many external script files all contribute to poor INP scores.
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