Third party scripts

Third-party scripts — tag managers, ads, analytics, consent, chat, embeds — add JavaScript that can keep the main thread busy, delay rendering and delay interaction; web.dev advises using async or defer for any third-party script not needed for the critical rendering path, lazy-loading embeds and ads until they are needed, and adding them after the critical content loads. Measuring them has a limit: Resource Timing reports transferSize and body sizes as 0 for cross-origin resources unless the vendor sends Timing-Allow-Origin, so request counts per host are reliable and bytes often are not.