If TTFB jumps, LCP usually follows
Alert when the page gets slower.
Time to First Byte is how long the browser waits before the HTML starts. It is not a Core Web Vital on its own. It is often the reason LCP fails. A page cannot paint the hero in 2.5 seconds if the HTML took 2.1.
What makes TTFB move
- Cache bypass after a deploy
- Origin CPU during a sale
- A plugin that hits the database on every view
- Redirect chains (http to https to www to locale)
- Geographic distance from the user to the origin if you have no edge cache
How to watch it
Pick the URLs that make money. Record TTFB daily from a lab probe and, when you have it, the field p75. Alert on a jump, not on a single noisy run. A 400ms shift that stays for two days is a story. One 2s spike at 03:00 might be a backup window.
What to do when it jumps
Look at cache hit rate, origin logs and the redirect list. Do not start by compressing images. Images are not TTFB.
FAQ
Is a 200ms TTFB good?
For an HTML document that is a solid lab number. Field users on slow networks will be higher. Compare against yourself last week, not against a blog's "ideal".
Does CDN always fix it?
A CDN fixes TTFB when the HTML is cacheable. Personalized HTML still waits on origin.