Glossary
TTFB (Time to First Byte)
How long the server takes to start answering.
What it is
Time to First Byte measures the delay between the browser requesting a page and receiving the first byte of the response. It covers DNS, connection, and — the part that usually matters — how long your server spent building the page.
Why it costs you money
TTFB is the first thing to check on a slow store, because it separates a server problem from a browser problem. If it is poor on uncached pages, no amount of frontend work will help: the bottleneck is hosting, PHP capacity or the database.
How to check it yourself
Measure it on a page that cannot be cached, such as cart or a logged-in view. Comparing it against your LCP and INP tells you which half of the stack to spend money on.
Where this matters