Performance and Core Web Vitals

you were handed three numbers and told the page fails on one of them, at the 75th percentile.

Core Web Vitals

the browser sat waiting with an empty tab before a single byte of HTML came back.

Time to first byte

the first text or image appeared, so the user finally knew the page was alive.

First contentful paint

the big hero image finally rendered, four seconds after the header was already on screen.

Largest contentful paint

you went to click the button, an ad loaded above it, and you clicked something else entirely.

Cumulative layout shift

you clicked the checkbox and it took most of a second before the tick appeared.

Interaction to next paint

the lab report says two seconds of blocking time and you can see it as a wall of yellow in the profile.

Total blocking time

the page looked finished for a while before it would actually respond to a click.

Time to interactive

one function held the main thread for 300 milliseconds and every click during it queued up behind it.

Long task

the scroll is smooth and then jumps, because a frame took too long and got skipped.

Jank

you have about ten milliseconds of your own work per frame before the animation starts dropping.

Frame budget

you read `offsetHeight` and wrote a style inside the same loop, and the loop got slower with every item.

Layout thrashing

the profiler flags a purple block with a warning triangle where you asked for an element's width.

Forced synchronous layout

you told the browser to skip rendering the sections that are off screen and the initial layout got much faster.

content-visibility

you ran the audit on your laptop, got a score of 98, and shipped a page that is slow for everyone else.

Lab data

the dashboard shows the metric at the 75th percentile across everyone who actually loaded the page last month.

Field data

you moved the number from 62 to 91 and nobody could feel the difference on the actual site.

Lighthouse score

you showed a response to the click immediately, even though the real work took another second.

Perceived performance