Alkir I don't think deferring javascript makes any difference in Flarum.
As we don't have any server-side rendering for the single page app, nothing is visible until the javascript has finished running, whether it's synchronous or asynchronous.
I think deferring the javacript could even make things worse because it would remove the browser blocking page interaction. On slow connections, the only thing users would see is the Flarum spinner and the browser wouldn't show it's still loading the page in the same way as currently. Users might think the loading is stuck whether now there might be a few more indicators that the page is being loaded by the browser.
