The key differences between asynchronous and synchronous ad code are:
Speed: Asynchronous ad code offers improved web latency, especially on mobile.
Improved user experience: Using asynchronous ad code ensures your users see your content, even if they’re having trouble loading your ads.
The advantage of the asynchronous code is that it’s guaranteed never to get in the way of other parts of your web page loading. This means that users will be able to load the content of your pages, even if they’re having trouble loading the ads. Mobile users in particularly will enjoy less latency when loading web pages.