The Core Web Vitals, and which of them a backend affects

Three metrics became a ranking signal in 2020 and two of them are frontend problems, which is worth knowing before a backend team is asked to fix all three.

LCP  largest contentful paint. BACKEND AFFECTS THIS.
     server response time is a direct input. so is
     whether the hero image is discoverable early.

FID  first input delay. main-thread JavaScript. not you.

CLS  cumulative layout shift. images without dimensions,
     injected banners, web fonts. not you, mostly.

TTFB is not a vital and is the thing a backend controls.

Time to first byte is the input the backend owns and it is not itself one of the three, which makes the conversation confusing — improving TTFB by 300ms improves LCP by 300ms and moves neither of the others. Saying which metric each team can move is the useful contribution to the meeting. The 75th percentile of real user data is what counts, not a lab measurement, which is why a fast local test proves nothing.