Speed isn't just a metric; it's a feature. Google's Core Web Vitals measure the experience of your site, not just the code.
1. Largest Contentful Paint (LCP)
What it is: How long it takes for the main content (usually the hero image or H1) to appear. Goal: Under 2.5 seconds. Fixes: Optimize images (WebP/AVIF), use CDNs, and prevent "render-blocking" JavaScript (defer non-essential scripts).
2. Interaction to Next Paint (INP)
(Note: INP replaced FID in March 2024) What it is: Responsiveness. When a user clicks a button, does the browser freeze or respond instantly? Goal: Under 200 milliseconds. Fixes: Break up long JavaScript tasks. If your main thread is blocked calculating Fibonacci numbers, it can't handle a click event.
3. Cumulative Layout Shift (CLS)
What it is: Visual Stability. Does the text move while you're reading it because an ad loaded above it?
Goal: Less than 0.1.
Fixes: Always define width and height attributes on images and videos. Reserve space for ads before they load.
The Business Case
Walmart found that for every 1 second of improvement in load time, conversions increased by 2%. Users don't wait. If your LCP is 5 seconds, they've already hit the "Back" button and gone to your competitor.