v132 · performance

Expose coarsened cross-origin renderTime in elment timing/LCP (regardless of TAO)

All element-timing and LCP performance entries would have a non-zero renderTime, even if they are cross-origin without Timing-Allow-Origin. All presentation timestamps (renderTime, paint timing start time, event timing end time) will be coarsened to a 4ms multiple to mitigate the risk of reading cross-origin image information.

concepts

  1. Coarsened renderTime

    Meaningful LCP for third-party hero images without TAO.

  2. TAO vs coarsened

    Load two hosts side-by-side — one serves Timing-Allow-Origin, the other doesn't — and compare reported renderTime precision.

  3. 4 ms grid visualiser

    Slide a raw renderTime; the second axis snaps it to the 4 ms grid the browser actually reports. See exactly which bucket every microsecond falls into.

  4. live LCP table

    A real hero element. Swap the image and watch new largest-contentful-paint entries arrive in the table, including the v132 coarsened renderTime field.

why it shipped

Element timing and LCP entries have a "renderTime" attribute, aligned with the first frame in which an image or text was painted.

references