Lesson 3 / 25
Performance metrics fundamentals
Learn FCP, TTFB, FMP, and other key metrics that measure different aspects of loading and rendering.
Time to First Byte (TTFB)
TTFB is the time from request to first byte of response. Includes network latency and server processing. Target: < 600ms. Optimize server response time and use CDN to reduce this metric.
First Contentful Paint (FCP)
FCP marks when first content (text, image, or background) appears. Target: < 1.8 seconds. Critical for perceived load performance—users want to see something quickly.
Time to Interactive (TTI)
TTI is when the page becomes fully interactive. Heavy JavaScript can delay this. Focus on reducing main thread blocking and ensuring responsive interactions after content loads.
Metric relationships
Timeline: TTFB → FCP → LCP → TTI. Each metric builds on previous. Understanding this sequence helps identify bottlenecks. A high TTFB affects all downstream metrics.
Quick check
Quick check: What does TTFB measure?
- Time to first response byte
- Total transfer time
- Time to finish loading