JS & CSS Performance Budget Calculator
Heavy scripts block rendering. Calculate your site’s “Time Cost” based on file size and connection speed. See if you fit within the Core Web Vitals budget.
The Hidden Cost of JavaScript
File size isn’t just about download time. With JavaScript, there is a second cost: Execution Time.
When a browser downloads 1MB of JavaScript, it has to unzip it, parse it, compile it, and execute it. On a powerful MacBook, this takes milliseconds. On an average Android phone, this can block the CPU for 2-3 seconds, freezing the screen completely.
This Performance Budget Calculator estimates both the network cost and the CPU cost of your code, helping you stay within Google’s Core Web Vitals limits.
What is a Performance Budget?
A performance budget is a limit you set for your team (e.g., “Our total JS cannot exceed 200KB”). It prevents “feature creep” from destroying your site speed.
- Interaction to Next Paint (INP): Measures responsiveness. Heavy JS forces the browser to ignore user clicks, ruining this score.
- Total Blocking Time (TBT): The total time the main thread is blocked (unable to respond) during load. Ideal is less than 200ms.
3 Ways to Reduce Script Weight
“Shaking” the code tree to remove dead leaves. This process (used in Webpack/Vite) automatically removes unused functions from your final bundle.
Don’t load the “Checkout” code on the “Homepage.” Split your code into chunks and load them only when needed (Lazy Loading).
Defer and Async
Never put scripts in the <head> without attributes.
- async: “Download in background, execute as soon as ready.” Good for analytics.
- defer: “Download in background, execute only after HTML is parsed.” Good for everything else.
Pass Core Web Vitals
Code bloat is silent but deadly. We perform deep performance audits to minify, defer, and optimize your assets for lightning-fast load times.
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