At a glance the headline numbers for Iframes per page
How many iframes the page embeds.
The typical page embeds 0 iframes.
Distribution & median CLS site count and median CLS at each level of iframes per page
Passing CLS by iframes per page which level passes the CLS most often
Iframes per page 0. p75 2. p99 8. At the low end (0–0): CLS 0.00. At the high end (3+): CLS 0.00. computed
Why this matters for the Core Web Vitals, and where to start fixing it
Each iframe is a separate document with its own resources, scripts and lifecycle, and its work still competes for the same device. The count is a decent proxy for embed weight: chat widgets, videos, ad slots, forms.
Every embed should justify itself. The heavy default (always-on iframe) has a light replacement for almost every case: a facade for video and chat, a static image for maps, a reserved slot for ads.
How does this affect the Core Web Vitals?
Iframes correlate with the CLS. With few iframes, 91% of sites pass the CLS. With many, 83% do. The decline is gradual. There is no point where sites suddenly start failing.
Chrome field data from 189,915 sites, representing millions of real page loads. How we measured.
Live queries (2) — admin only
SELECT COUNT(*) AS count,
quantile_disc(m.iframes.total, 0.10) AS p10,
quantile_disc(m.iframes.total, 0.25) AS p25,
quantile_disc(m.iframes.total, 0.50) AS p50,
quantile_disc(m.iframes.total, 0.75) AS p75,
quantile_disc(m.iframes.total, 0.90) AS p90,
quantile_disc(m.iframes.total, 0.99) AS p99
FROM sites WHERE m.iframes.total IS NOT NULL;SELECT CASE WHEN m.iframes.total >= 0 AND m.iframes.total < 1 THEN 0 WHEN m.iframes.total >= 1 AND m.iframes.total < 2 THEN 1 WHEN m.iframes.total >= 2 AND m.iframes.total < 3 THEN 2 WHEN m.iframes.total >= 3 THEN 3 END AS bucket_idx, COUNT(*) AS n,
quantile_disc(crux."phone".cls, 0.5) AS median,
COUNT(*) FILTER (WHERE crux."phone".cls IS NOT NULL AND crux."phone".cls <= 0.1) * 1.0 / NULLIF(COUNT(*) FILTER (WHERE crux."phone".cls IS NOT NULL), 0) AS good_pct
FROM sites WHERE m.iframes.total IS NOT NULL GROUP BY bucket_idx;