Time to First Byte (TTFB)

Time to First Byte measures the gap between a request leaving the browser and the first byte of the response arriving. It is upstream of everything else: no matter how well optimised your images and scripts are, the page cannot begin rendering until that first byte lands.

A good TTFB is under roughly 800 milliseconds. Slow scores usually point to the back end: an overloaded server, heavy or uncached database queries, slow third-party calls, or hosting that sits far from the visitor.

The most reliable fix is to take the server off the critical path. Static pages served from a CDN respond in tens of milliseconds because there is no database query or template render at request time. This is a large part of why moving a heavy, dynamic site to a static stack produces such an immediate speed jump.