Partytown is an open-source JavaScript library, developed by Builder.io and released in 2022, that relocates resource-intensive third-party scripts — such as analytics, tag managers, and advertising pixels — from the main browser thread into a web worker, preventing them from blocking the main thread and degrading Core Web Vitals metrics such as Total Blocking Time and INP. Because web workers cannot access the DOM directly, Partytown uses a synchronous proxy approach: the web worker communicates with the main thread via a shared SharedArrayBuffer or a service worker fallback to intercept DOM reads and writes transparently. The script tag is replaced with a type="text/partytown" attribute, which Partytown picks up and executes in the worker context. While it can substantially reduce main-thread contention, the synchronous communication overhead means it works best for scripts that do light DOM interaction. Partytown ships as a framework-agnostic library and has official integrations with Next.js, Nuxt, Astro, Gatsby, and Remix.