Web Performance Glossary

Largest Contentful Paint

When the biggest visible element on the page finishes loading.

First review: Review the LCP element path: image size and format, preload or fetch priority, and CSS that blocks rendering.

Cumulative Layout Shift

How much content jumps around during loading.

First review: Reserve stable space for the element that moves before changing unrelated scripts or styles.

Interaction to Next Paint

How quickly the page responds to taps and clicks.

First review: Reduce the work triggered by the slowest user action before trimming unrelated JavaScript.

Core Web Vitals

Google's main page experience metrics: LCP, INP, and CLS.

First review: Pick one failing metric with the clearest evidence and fix that bottleneck before chasing the overall score.

Time to First Byte

How long the server takes to start responding.

First review: Review redirects and public-page caching before tuning images, fonts, or frontend scripts.

First Contentful Paint

When the first text or image appears.

First review: Make above-the-fold text or shell content render without waiting on non-critical CSS, JavaScript, or late data.

Total Blocking Time

How long scripts freeze the browser (lab only).

First review: Split or defer the script work that blocks the main thread during the first load.

Speed Index

How quickly visible content fills the screen.

First review: Prioritize above-the-fold rendering and defer below-the-fold assets or scripts that delay visual progress.

Time to Interactive

When the page becomes fully usable.

First review: Reduce startup and hydration JavaScript that runs after visible content appears.