JS Execution Time Test
The JavaScript Bootup Time Test measures the total time the browser spends parsing, compiling and executing JavaScript during page load.
- Results in seconds
- Pass / fail + fix guidance
- No account required
The JavaScript Bootup Time Test measures the total time the browser spends parsing, compiling and executing JavaScript during page load. High bootup time blocks the main thread, delays Time to Interactive, and directly hurts your Interaction to Next Paint score. The report breaks bootup time down per script so you can target the biggest offenders — usually third-party tag managers, oversized first-party bundles, or heavy framework hydration.
What This Tool Checks
- Total JavaScript bootup time
- Per-script parse / compile / execution breakdown
- Third-party JavaScript bootup contribution
- Long tasks (>50 ms) caused by JavaScript
- Total bytes of JavaScript transferred
- Code-splitting and tree-shaking opportunities
Why It Matters for SEO
JavaScript is the most expensive resource a browser handles — far more than equivalent bytes of HTML, CSS or images. Every script must be parsed, compiled and executed, all on the main thread, all before user interaction can be processed. High bootup time is the root cause of slow TTI, poor INP and unresponsive interactions. The fix is to ship less JavaScript: code splitting, tree shaking, removing third parties, and adopting islands architecture.
How to Fix It
Code-split per route. Tree-shake unused dependencies. Audit third-party tag managers and remove what is not driving clear value. Use the islands architecture for SSR pages so only interactive components hydrate. Audit polyfills against modern browser support and remove what is no longer needed.
How It Works
Headless Chrome captures the JavaScript execution timeline during page load via the Performance API, attributes each parse / compile / execute slice to its source script, and totals the time. Scripts longer than 200 ms of bootup time are flagged with concrete reduction options.
Common Mistakes to Avoid
- Loading the entire app bundle on every page (no code splitting)
- Including polyfills for browsers that no longer need them
- Tag managers that load dozens of small scripts
- Server-side rendering followed by full client-side hydration of static content
- Including dev-mode bundles in production
Quick Checklist
- JavaScript bootup under 2 seconds on mobile
- JavaScript transferred under 200 KB per route
- No single script over 50 KB minified
- Third-party scripts audited and minimised
- Hydration scoped to interactive islands
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The total time the browser spends parsing, compiling and executing JavaScript during page load. High bootup blocks the main thread, delays interactivity and hurts INP.
A practical target is under 200 KB transferred and under 2 seconds of bootup time on mid-range mobile. Above that you start losing INP and TTI scores measurably.
Bytes-for-bytes, JavaScript is roughly 10x more expensive than equivalent HTML/CSS/image bytes. Every script must be parsed, compiled and executed on the main thread before it can affect the page.
Code-split per route, lazy-load components below the fold, tree-shake unused exports, audit and remove third parties, and use islands architecture to ship JS only where interaction needs it.
Indirectly but powerfully. High bootup hurts INP and TTI, which feed Google's page-experience ranking signal. Reducing bootup almost always improves Core Web Vitals scores.