Is Start Render Time (Lab) same as FCP (CrUX)

1

Is renderStart in summary_pages table same as FCP.? Appears FCP is derived from _chromeUserTiming.firstContentfulPaint from the CrUX dataset (field data) whereas renderStart is from lab measurements; conceptually they appear similar. Requesting help in understanding the difference between the two metric.

The definitions are as follows:

a) The Start Render time is measured as the time from the start of the initial navigation until the first non-white content is painted to the browser display (https://sites.google.com/a/webpagetest.org/docs/using-webpagetest/metrics)

b) First Contentful Paint (FCP) measures the time from navigation to the time when the browser renders the first bit of content from the DOM. (https://developers.google.com/web/tools/lighthouse/audits/first-contentful-paint?utm_source=lighthouse&utm_medium=unknown)

1 Like
2

I think render start is more closely aligned with “First Paint”. @patmeenan could you confirm?

3

FCP, first paint and start render are all slightly different but in practice for the vast majority of sites they end up being the same (within measurement error). FCP and first paint are when chrome thinks it painted content. Start render is observed from the outside and is when the viewport actually changed (within 100ms).