Time to First Byte

Time to First Byte (TTFB) is the measurement of how long a browser waits to receive the first byte of data from a server after making an HTTP request. It represents the sum of redirect time, DNS lookup, server processing, and network latency before content begins downloading.

Meaning & Context

TTFB is a critical server-side performance metric that indicates backend efficiency and hosting quality. It includes the time for the request to reach the server, the server to process the request and generate a response, and the first byte of that response to travel back to the browser. Industry standards suggest TTFB should be under 200-300 milliseconds for optimal performance.

Why It Matters

In A/B testing and CRO, TTFB directly impacts page load speed, which significantly affects conversion rates and user engagement. A slow TTFB can undermine even the best-designed test variations by creating poor user experiences. Additionally, if test variations have different TTFB values due to server-side implementation, it can confound results by mixing content changes with performance differences.

Example

An A/B test shows the control page converting 5% better than a variation, but analysis reveals the variation's TTFB is 800ms compared to the control's 200ms. The performance difference, not the design changes, likely caused the conversion drop, invalidating the test conclusions about the design elements being tested.