When it comes to maximizing the revenue of your website there are many aspects of optimization that we consider. These aspects such as page layout, content quality, traffic sources, and others. But there are also other factors that can be controlled using the features on your ad server (Google Ad Manager / DoubleClick for Publishers – DFP)
Synchronous and asynchronous rendering are the mods google provides to load ads on your website, asynchronous provides the best page load experience for the users. This is due to the fact that asynchronous will allow your website content to load independently from threads. In async mode the page will continue to load regardless of the time it will take to load a heavy creative ad and thus users won’t have to wait for the ads to load to see the content they are looking for. However, synchronous rendering will load all the content on the page according to the order that the browser fetches them, this means that if there is a heavy creative that takes time to load a user will have to wait for the ad to load before he/she can see the rest of the content on the page. However, this mod of rendering is a better choice for pages with multi-sized ad units as the content will only load after the ad has been served and will allow the layout to adjust according to the size of the ad. It is also a required method when you are running passback tags.
Asynchronous is highly recommended along with the Single Request Architecture (SRA)option enabled in Ad Manager/DFP because it provides better loading times and the potential to serve guaranteed roadblocks. In the end, it all comes down to what you prefer for your website whether it is giving your visitors a better experience by choosing asynchronous rendering or if you prefer having multisize and passback ads running in your inventory by implementing synchronous rendering mode which will also guarantee that your layout stays intact.