At a glance the headline numbers for Media source
Where media files come from: same origin vs third party.
35.2% of media files stream from third parties.
The media source mix who uses what, and how stable each group is
Media source. On the fleet: 64.8% self, 35.2% third party. 6.7% of sites use at least one self.
Self leads by count (64.8%) and by bytes (72.4%). computed
Passing CLS per bucket every category and count level at once - color is the pass rate
Each row is a category, each column its own count bucket (few on the left, many on the right); the cell is the share of those sites passing CLS.
Third party swings the hardest: 78% of sites pass CLS with few, 69% with many. computed
Few vs many - does quantity cost CLS? the pass rate with few vs many of each category
Per category: the pass rate among pages with FEW of it (hollow ring) against pages with MANY (solid dot), worst trend first. Thin buckets are excluded from the endpoints.
More Third party costs the most: the CLS pass rate falls from 78% with few to 69% with many. computed
Why this matters for the Core Web Vitals, and where to start fixing it
Video files are the biggest assets a page can carry, and the source decides who controls their delivery: the cache rules, the compression, the range requests that make seeking work. Third-party video also means another connection, usually for the largest download on the page.
For embedded players the source question disappears behind the iframe, and the facade pattern beats both options: a poster image until the visitor clicks. Host video yourself only with real video infrastructure behind it. Adaptive bitrates do not happen by accident.
How does this affect the Core Web Vitals?
The choice barely moves the LCP: 86% pass at best, 85% at worst. 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.