<script defer=”defer” src=www.example.com/somescript.js>
signals that the script can be loaded in parallel with the main document. The advantage is that a slow service from Google or other ad provider does not have to have an adverse effect on overall page render times.
by loading all the parts separately means that a browser can assemble the page in parallel. This is particularly important with slower internet connections seen with mobile devices.
Using the defer tag can improve page load time by 500ms or more. Faster the page load, the better.