Page Speed Glossary: Every Metric Explained
A comprehensive glossary of every page speed metric, acronym, and concept you need to know. Bookmark this reference guide.
Web performance has its own vocabulary. This glossary covers every metric, acronym, and concept you'll encounter when optimizing page speed.
Core Web Vitals
LCP — Largest Contentful Paint
What: The time it takes for the largest visible content element (image, video, or text block) to render in the viewport. Good: ≤2.5 seconds Poor: >4.0 seconds How to improve: Optimize the LCP resource (usually hero image), reduce TTFB, eliminate render-blocking resources.
INP — Interaction to Next Paint
What: Measures how quickly the page responds to user interactions (clicks, taps, key presses). Reports the worst interaction latency (at the 98th percentile). Good: ≤200ms Poor: >500ms How to improve: Reduce JavaScript execution time, break up long tasks, minimize main thread work. Replaced: FID (First Input Delay) in March 2024.
CLS — Cumulative Layout Shift
What: Measures unexpected layout shifts during the page's lifetime. A layout shift occurs when a visible element changes position without user interaction. Good: ≤0.1 Poor: >0.25 How to improve: Set image/video dimensions, reserve space for dynamic content, avoid inserting content above existing content.
Performance Timing Metrics
TTFB — Time to First Byte
What: Time from the user's request to the first byte of the response arriving from the server. Good: <500ms Includes: DNS lookup + TCP connection + TLS handshake + server processing + network latency.
FCP — First Contentful Paint
What: Time until the browser renders the first piece of content (text, image, canvas, SVG). Good: <1.8 seconds Note: Not necessarily meaningful content — even a loading spinner counts.
FID — First Input Delay (Deprecated)
What: Time from the user's first interaction to when the browser can respond. Status: Replaced by INP in March 2024. CrUX no longer reports FID.
TBT — Total Blocking Time
What: Total time between FCP and TTI where the main thread was blocked for more than 50ms (preventing input response). Good: <200ms Relationship: Lab equivalent of INP/FID. Correlates with real-world responsiveness.
TTI — Time to Interactive
What: Time until the page is fully interactive (main thread idle for 5+ seconds after last Long Task). Status: De-emphasized by Lighthouse. TBT is now preferred.
SI — Speed Index
What: How quickly content is visually populated in the viewport. Measured in milliseconds. Good: <3.4 seconds How it works: Captures video of loading, calculates visual completeness over time.
Resource Metrics
Total Byte Weight
What: Total size of all resources downloaded by the page (HTML, CSS, JS, images, fonts, etc.). Target: <1.5MB for optimal mobile performance.
DOM Size
What: Total number of HTML elements (nodes) in the page. Target: <1,500 nodes. Excessive DOM size affects style calculation and layout performance.
Long Tasks
What: Any JavaScript task that runs for more than 50ms on the main thread, blocking user input. Impact: Each Long Task can cause dropped frames and input delay.
Lighthouse Scores
Performance Score (0-100)
Weighted combination of:
- FCP: 10%
- SI: 10%
- LCP: 25%
- TBT: 30%
- CLS: 25%
Accessibility Score (0-100)
Automated checks for WCAG compliance: color contrast, alt text, ARIA labels, heading structure, etc.
Best Practices Score (0-100)
Security, modern web standards, and error-free console.
SEO Score (0-100)
Basic SEO requirements: meta tags, crawlability, mobile-friendliness.
Rendering Concepts
Critical Rendering Path
The sequence of steps the browser takes to render the page:
- Parse HTML → build DOM
- Parse CSS → build CSSOM
- Combine DOM + CSSOM → Render Tree
- Layout (compute geometry)
- Paint (draw pixels)
- Composite (layer management)
Render-Blocking Resources
CSS and synchronous JavaScript in <head> that prevent the browser from rendering content until they're fully loaded.
Above the Fold
Content visible in the viewport without scrolling. Prioritize loading above-the-fold content first.
Hydration
The process where a JavaScript framework attaches event listeners and state to server-rendered HTML, making it interactive.
Optimization Techniques
Lazy Loading
Deferring the loading of non-critical resources until they're needed (usually when they enter the viewport).
Code Splitting
Breaking JavaScript bundles into smaller chunks that load on demand, rather than loading everything upfront.
Tree Shaking
Removing unused code from JavaScript bundles during the build process.
Preloading
Telling the browser to fetch critical resources early: <link rel="preload" href="font.woff2" as="font">.
Prefetching
Telling the browser to fetch resources for future navigation: <link rel="prefetch" href="/next-page.js">.
Preconnecting
Establishing early connections to third-party origins: <link rel="preconnect" href="https://cdn.example.com">.
Infrastructure Terms
CDN — Content Delivery Network
A network of servers worldwide that cache and serve your content from locations close to users, reducing latency.
Edge Computing
Running code at CDN edge locations (close to users) instead of a centralized server.
SSR — Server-Side Rendering
Rendering HTML on the server for each request, sending complete HTML to the browser.
SSG — Static Site Generation
Pre-rendering HTML at build time, serving static files from a CDN.
ISR — Incremental Static Regeneration
A hybrid approach where static pages are regenerated on a schedule or on-demand.
Monitoring Terms
Synthetic Monitoring
Running automated tests from controlled environments (lab data). Consistent but doesn't represent real users.
RUM — Real User Monitoring
Collecting performance data from actual user sessions (field data). Represents real experience but varies by device/network.
CrUX — Chrome User Experience Report
Google's dataset of real-user performance data collected from Chrome users. Used for Core Web Vitals ranking signal.
Performance Budget
Pre-defined limits on performance metrics that trigger warnings or block deployments when exceeded.
Bookmark This Reference
Come back to this glossary whenever you encounter an unfamiliar performance term.
Ready to stop wasting ad spend?
Track your landing page performance, monitor Core Web Vitals, and calculate exactly how much slow pages cost you.
Start Free — No Credit Card