User blog comment:Rappy 4187/Technical Update: October 19, 2016/@comment-32881089-20161021182104/@comment-11733175-20161022093104

The code snippet that generates a warning is the core jQuery library or, more specifically, the $.ajax functions which support both synchronous (meaning in series) and asynchronous (meaning in parallel) HTTP requests. It's up to the person using the library to use it as they see fit. The vast majority of requests on Fandom are asynchronous.

The document.write warning is due to how MediaWiki tries to handle loading of scripts before the DOM can be manipulated. It's meant to load script synchronously, but document.write is asynchronous and it never worked. Interestingly, this was fixed in a later version of MediaWiki just over a year ago and claimed to improve page load by 40%. I don't know if the same could be said of Fandom's performance, but it would be something interesting to look into.