Synchronous and Asynchronous JavaScript Tags

The use of synchronous or asynchronous JavaScript (JS) tags determines the order of how the page is generated and impacts loading speeds. Where there is a lot of JavaScript activity, this can really slow down rendering of a mobile site and weaken the user experience.

To contribute to instantaneous loading times, Marfeel employs asynchronous JS tags, which has played a key role in improving page speed from 5-7 seconds to under 1 for one customer or achieving a 95/100 Google Score for another.

Asynchronous tags

Asynchronous JS tags allow scripts and elements to be loaded simultaneously in a non-blocking way so the user is never left waiting due to a long cascade of elements that have to load, one after the other. All blocking elements are removed so everything can be loaded at the same time. 

Synchronous tags 

Synchronous JS tags are designed to sequentially load scripts, one after the other, starting with the <head> tag. With synchronous tags, all subsequent scripts can't be loaded until the one before them has finished, thereby blocking other elements from loading. 

This can cause serious rendering delays and significantly slow down load times.  

Synchronous tags cannot be used on the Marfeel platform.

Interstitial Ads

Most of the time, synchronous JS tags are necessary for interstitial ads because they need to be loaded before other elements of a mobile site. This is just one of the reasons Marfeel strongly discourages the use of interstitals, especially if they require synchronous tags.

The use of sync tags will also significantly undermine a mobile site's Google PageSpeed score. 

Advantages of asynchronous tags

Asynchronous tags significantly improve overall load time. By allowing elements and scripts to be executed simultaneously, the user won't experience rendering delays common with the use of synchronous JS tags. 

Asynchronous tags are also what allow us to provide our engaging UX elements like the user scroll which increases reading time and impressions.