Most “performance projects” start with images and fonts. Fair enough. But the pages that still feel bad after a hero image is optimised, preloaded, and served from a CDN are often suffering from a different problem: third-party JavaScript. Tag managers, chat widgets, A/B tests, and social embeds do not have to be evil. Left unmanaged, they absolutely compete with your first paint, your LCP , and the interactions that INP measures. This guide is a practical way to name the worst offenders, decide what to cut or delay, and prove the win before the marketing team files a ticket because “the numbers look wrong”. What “third party” really means in performance work A third-party script is any JavaScript, iframe, or stylesheet you did not write that loads from another origin: analytics, ads, reviews, personalisation, consent platforms, and the tag manager that wires them together. Browsers do not care whether a script is “small business friendly”.…