Asynchronous & Defer loading
Asynchronous loading allows scripts to load at the same time. It downloads site files as usual, but pauses the execution until the process is complete. While this can be faster than sequential load set ups, it may also cause the website to flicker from its original version to the expected version at the start.
Defer loading delays the script execution until all of the HTML for the website is parsed and displayed. The files are likewise downloaded during normal parsing, but executes them only after the parser has completed. They are executed in the order they appear in the document. This can make the website content load much faster, although the site visitor will have to wait for the entire thing to become operational. Time difference included with using the defer attribute is minuscule, of course, but all potential delays at up.
Asynchronous and defer loading happens when the web developer inserts a simple attribute in the script call lines in the HTML file’s head section: either async or defer. The decision about which one to use or whether you should use either one at all comes down to several factors. Things like script element location, file type, and dependency all matter. Find more information about asynchronous and defer loading here.