For users on slow connections such as 2G, the performance penalty from third-party scripts loaded via document.write() is often so severe as to delay display of main page content for tens of seconds. This feature will block the load of cross-origin, parser-blocking scripts inserted via document.write() in case of an HTTP cache miss for users on a 2G connection. The feature will only be applicable to such scripts in the main frame.
Docs: https://developers.google.com/web/updates/2016/08/removing-document-write https://developer.mozilla.org/en-US/docs/Web/API/Document/write https://docs.google.com/document/d/1dMJRQKTw75ZNdknP3pirSBH3koPl_IWHnxlcBuu4t_c/edit?usp=sharing https://docs.google.com/document/d/1FSegTIN91btRwPCIrt6s3X63cdOJmNjcr0p2mhlOAW8/edit?usp=sharing https://docs.google.com/document/d/1AtS83d9joGj-xmt_pZB4Uvvn_9d1aEReRg0rI_5runw/edit