I read this nice article on how external scripts block the UI thread but it wasn\'t clear to me whether the blocking is actually due to the presence of the
Any loading of a JS file or any execution of any JS (whether in an external file or inline) will block the UI thread.
The exception for the tag is an asynchronous load where the script will load and execute asynchronously in the background.
There are also "deferred" loads (i.e. the defer
attribute) which tells the browser not to actually execute the JS therein until the rest of the page has loaded.