Time to First Byte, or TTFB, measures the gap between a browser sending a request and receiving the very first byte of the response back, covering DNS lookup, connection setup, and however long the server takes to actually process the request and start responding. Nothing else on the page can start loading until this step finishes, which makes it the true floor under every other speed metric.
A slow TTFB, generally anything much above 600-800 milliseconds, points to server-side or infrastructure problems rather than anything about the page's own front-end code: an overloaded or distant server, slow database queries, uncached dynamic content being rebuilt on every request, or a hosting plan that's simply undersized for the traffic it's serving. Because every later metric, First Contentful Paint, Largest Contentful Paint, depends on TTFB finishing first, a slow TTFB drags the whole loading sequence down with it regardless of how well-optimized everything downstream is. Server-side caching, a CDN serving content from a location closer to the visitor, and addressing genuinely slow backend queries are the standard fixes, since none of them touch the front-end code a page-speed audit might otherwise focus on first.
