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Lesson 3 / 25

Performance metrics fundamentals

Learn TTFB, FCP, TTI 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 reliably interactive. Heavy JavaScript delays it. Lighthouse no longer scores TTI, so use Total Blocking Time (TBT) in the lab and INP in the field. Reduce main-thread blocking either way.

Metric relationships

Typical timeline: TTFB → FCP → LCP, with interactivity measured by TBT and INP. Each metric builds on the previous one, so a high TTFB affects everything downstream.

Quick check

Quick check: What does TTFB measure?

  • Total transfer time
  • Time to finish loading
  • Time to render the largest image
  • Time to first response byte
Answer

Time to first response byte — TTFB covers network latency plus server processing until the first byte arrives.