Third-party domains

Third-party domains are a common Core Web Vitals issue. First of all, each third-party domain needs a new connection. That means time is lost on a DNS lookup, a TCP handshake and a TLS negotiation. If you had served those resources from your main domain, the browser could have re-used the connection that is already open (that is why you want HTTP/3: one fast, re-usable connection for everything you control). When that domain serves a font or a stylesheet, the browser cannot render before the connection is ready. The connection setup delays the LCP.

Also, third-party resources are usually out of your control. They can become a SPOF (single point of failure) when their server is slow or down. They can also change in size or behaviour without your knowledge, and your Core Web Vitals change with them. Self-hosting fixes both problems at once. Preconnect only hides part of the connection cost.

How do third-party domains affect the Core Web Vitals?

Third-party domains correlate with the LCP. With a single third-party domain, 89% of sites pass the LCP. Among the sites with the most third parties, 81% do. The decline is gradual. There is no point where sites suddenly start failing.

The effect is bigger on CLS. With a single third-party domain, 93% of sites pass it. Among the sites with the most third parties, 78% do. Third parties inject ads, embeds and consent banners, and injected content moves the layout after the page looks done.