Blocking frame breakdown
The blocking animation frame behind that interaction: script, forced reflow, style/layout, other (lab).
At a glance the headline numbers for Blocking frame breakdown
The blocking animation frame behind that interaction: script, forced reflow, style/layout, other (lab).
Script accounts for 73ms of the typical blocking frame.
Where the milliseconds go the phases at the median, against the budget
At the median the whole stack uses 79ms, inside the 200ms INP budget. The biggest piece is Script ms at 73ms. computed
Passing LCP per phase the share of sites passing, by how long each phase takes
Each line is one phase: left = sites where it is quick, right = sites where it drags (buckets, short to long). The steeper the fall, the more that phase decides LCP.
Forced reflow ms falls hardest: from its fastest to its slowest bucket the LCP pass rate drops 19 points, down to 65%. computed
The 4 distributions how each phase spreads across sites - color is the share passing in that range
Script ms stays green across its whole range: it never decides LCP. Forced reflow ms turns red in its tail - the failing sites live there. computed
Why this matters for the Core Web Vitals, and where to start fixing it
The long animation frame behind a slow interaction splits into parts. Script time is JavaScript running. Style and layout time is the browser recomputing the page after changes. Forced reflow is the expensive special case: a script writes to the DOM, then reads a layout value back, and the browser must stop mid-script and lay out the page to answer.
The split decides the fix. Script time wants less work or yielding. Style and layout time wants a smaller scope: a smaller DOM, containment. Forced reflow wants reordered code: batch the reads first, then do the writes.
How does this affect the Core Web Vitals?
Passing INP barely moves across the range: 96% at one end, 92% at the other. This signal does not separate passing sites from failing ones.
Chrome field data from 94,910 sites, representing millions of real page loads. How we measured.