Synchronous vs Asynchronous Rendering, which is best and when to choose which?

Ad Optimization
October 12, 2023 | by Aleesha Jacob

In AdTech, the battle between Synchronous and Asynchronous rendering is akin to a grand chess match, where every move impacts user experience and website performance. Synchronous rendering, with its structured and sequential approach, promises predictability and cohesiveness. In contrast, Asynchronous rendering champions speed and adaptability, ensuring users access content swiftly, irrespective of ad loads.

But which reigns supreme? Dive in as we dissect their strengths, weigh their limitations, and crown the ultimate victor for your digital strategy.

What happens to a page when synchronous rendering is used?

Synchronous mode makes the content and ads load depending on the order in which the browser reads them. This type of rendering is best for pages that use multi-sized ad units as the rest of the page content will load after the ad is served and adjust depending on its size, avoiding the possibility of a distorted site layout.

This is also the required mode when you’re running passback tags. In some instances where the ad didn’t return the ad request right away, the rest of the content won’t load as well which causes the user to wait. The result is a negative effect on the quality of the website experience.

Advantages of Synchronous Rendering:

1. Harmony with Multi-sized Ad Units:

When it comes to dynamic advertising, one challenge publishers face is the varying sizes of ad units. Synchronous rendering elegantly addresses this:

  • Predictable Layouts: By ensuring the ad loads first, the subsequent content adjusts around the ad’s dimensions. This means there’s no sudden “jump” or shift in the content, maintaining visual consistency.
  • Responsive Design: For web designs that are responsive to different devices, synchronous rendering ensures that the layout remains coherent, irrespective of the ad size displayed.

2. Compatibility with Passback Tags:

Passback tags are a fallback mechanism in online advertising. If the primary ad server cannot find a suitable ad to display, the passback tag redirects the request to another ad server. Given the sequential nature of synchronous rendering:

  • Sequential Call Sequence: The browser waits for the primary ad server’s response. If the primary server can’t fill the ad request, the passback tag triggers the secondary server without any overlap, ensuring a smooth transition.

Limitations of Synchronous Rendering:

Potential Delays in Content Presentation:

One of the main concerns with this method is its susceptibility to delays:

  • Dependence on Ad Server Response: If there’s a lag in the ad server’s response or if it takes longer to serve an ad, it can stall the loading of subsequent content.
  • User Experience Impact: A delayed loading might deter users who expect instantaneous content access, potentially increasing bounce rates.

What do Asynchronous tags enable us to do?

Unlike with synchronous, a user visiting pages that utilize the asynchronous mode gets to see the page content without having to wait for the page to display the whole content. The type of rendering mode loads pages independently from the ads.

The Mechanics of Asynchronous Rendering

In the asynchronous mode, the browser doesn’t adhere to a strict sequence for loading elements. Instead:

  • Parallel Loading: While one component (like text content) is loading, other elements (such as ads) can also begin their loading process concurrently.
  • Non-blocking Approach: Unlike synchronous rendering, where one element might block the loading of another, asynchronous rendering avoids these potential bottlenecks. Ads and content operate in their own independent lanes, ensuring one doesn’t interfere with the other’s loading time.


  • Enhanced Page Load Experience: Coupling asynchronous rendering with the Single Request Architecture (SRA) option in Google Ad Manager ensures a seamless page load.
  • Support for Ad Slot Refresh: This is the sole rendering mode supporting ad slot refresh.
  • Support for Large GET Requests: Previously, GPT GET requests over 4096 bytes faced truncation. The introduction of POST request method, supported only by asynchronous GPT tags, rectified this.
  • Buffer Against Slow Ad Servers: Even if an ad server is sluggish or unresponsive, the content remains unaffected and continues to load. This ensures that a hiccup in ad delivery doesn’t compromise the overall page experience.

Additional Insights:

  • SafeFrame Activation: Enabled by default with GPT tags, SafeFrame is favored over friendly iframes. While it safeguards site data from unauthorized access, it doesn’t impede data collection for ad relevancy.

It is highly recommended that asynchronous rendering and the SRA option in Google Ad Manager are enabled as they yield the best page load experience and the potential to serve guaranteed roadblocks as well as advertisers that competitors won’t serve their ads simultaneously to (competitive exclusion). This is the only rendering mode that makes ad slot refresh using the pub service. Refresh work.

Previously, GPT GET requests larger than 4096 bytes were not supported and were being cut short. To avoid truncation, they converted GET requests to POST request method which is now supported by asynchronous GPT tags only.

SafeFrame in Ad Manager is enabled by default when using GPT tags and recommended over-friendly iframes. It protects the sensitive data of the site from getting accessed by external elements but does not affect the data collection necessary to serve relevant ads to the site.

Considerations for Asynchronous Rendering:

While asynchronous rendering offers many advantages, it’s essential to:

  • Optimize Ad Placement: Given the parallel loading, strategic ad placement becomes vital to ensure ads are visible and effective.
  • Monitor Performance: Continuous monitoring can help identify if certain ads or content pieces are consistently slower, allowing for optimization.

Which Should You Choose?

Both rendering modes have their advantages and disadvantages. If you prefer to give the user experience more importance, choosing asynchronous rendering could be the best option. But in case you have multisize and passback ads running in your inventory, the synchronous rendering mode should be implemented to avoid ruining your site layout.

Advantages of Both Modes:

  • Synchronous Rendering:
    • Consistent Layout: It ensures that multi-sized ads fit perfectly, preventing unexpected shifts or distortions in your site layout.
    • Seamless Ad Integration: Especially crucial when using passback tags, this mode guarantees that ad requests are processed sequentially, eliminating overlaps or hitches.
  • Asynchronous Rendering:
    • Rapid Content Delivery: Users are treated to near-instantaneous content access, without being held hostage to ad load times.
    • Enhanced Resilience: It offers a robust defense against sluggish ad servers. Even if ads stall, the content marches on, undeterred.

Factors to Consider:

  • User Experience Priorities: If user satisfaction, marked by quick content delivery and seamless browsing, tops your list, asynchronous rendering emerges as the clear favorite.
  • Ad Complexity: For websites with a diverse ad inventory, especially those utilizing multi-sized ad units or relying heavily on passback tags, synchronous rendering provides the required structure and predictability.
  • Technical Infrastructure: Your website’s backend capabilities, server response times, and even factors like CDN integration can influence the efficacy of either rendering mode.
  • Future Scalability: Think long-term. As your website grows, will it see more complex ad integrations, or will you lean more towards enhancing user experience? Your growth trajectory can guide your choice.

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