IX Perspectives

Improperly implemented header bidding tags cause page slowdown and decreased revenue for publishers

These days, it seems like there is no such thing as a slow week in header bidding. Each new week brings new advancements, new learnings and new controversy. And this week has been no exception!

Since the advent and rise in popularity of header bidding wrappers, the new technology has been dogged by an undeserved reputation for increasing latency and hurting page performance. This reputation has been a source of constant confusion for those on my solutions team: time after time, we’ve seen the incredible improvements to page speed and increased ad revenue that the Header Tag Wrapper brings when properly set up by a team of skilled implementation engineers working closely with a client’s equally savvy dev team. That’s why it came as a surprise when we began seeing inconsistent behaviour with one of own wrapper implementations.

After hundreds of header implementations across the most sophisticated publisher environments, there was nothing out of the ordinary about these sites, nor our heavily road tested methodology. Why then were we not seeing the level of revenue uplift we have so many times before? Why were our integrated bidder peers seeing performance going down post-wrapper? Why were bids not making it from the wrapper into DFP when we were well under timeout?

After some deep dive analysis, we found that some header bidders operating outside of the wrapper were integrating into the header synchronously (or sync) by default, blocking javascript from running until their process was complete. Notably, this included both bidder code and the contents of the page. Google’s publicly available best practices for publishers refer to this type of synchronous tag as ‘render-blocking javascript’ and advise strongly against it. The industry standard has long been to implement any above the fold, external javascript calls asynchronously (or async), which allows processes to run simultaneously. This allows for the parallel auction that header bidding is known for and is the method that Index Exchange uses by default for all bidders in the Header Tag Wrapper. Running header tags synchronously has the potential to slow down pages, reduce revenue and even harm SEO.

Let’s take a closer look at how having a mixed header (sync and async) can confer unfair advantage and impact ad revenue:


500 ms timeout before DFP is called to render the ad slot


Partner A  – integrated asynchronously (industry standard) – needs 150 ms

Partner B  – integrated synchronously – needs 450 ms


Partner B consumes all resources for 450 ms, completes process

Partner A begins process only after Partner B completes, and gets cut off mid-process despite being 3x faster

Luckily, this issue is easy to both diagnose and resolve. To find out if you have a mixed header:

  1. Run a couple of your site URLs that serve ads and run header bidding through the Google PageSpeed Insights too. Note: Go beyond the homepage and ensure you are finding an article page that definitely has ad slots / header bidding active.
  2. Look for this section:

3. Click to expand “Show how to fix” in the “Eliminate render blocking JavaScript” section.

4. You will see this:

5. Look for the url of any ad tech vendors.

6. You can reach out to your ad tech vendor and request they upgrade their tags to be async rather than sync to prevent performance issues.

The client in question, who faced the same issue and observed sync tags in use by two 2 different header bidding vendors, worked with their partners to change the sync tags to async, and after only 2 days, their bid volume increased by nearly 25%:

At best, sync tags give a sub-optimally architected solution more time to complete. At worst, they can be seen to game the timeout mechanism, effectively holding back bidders who are following the rules asynchronously. Either way, the result is the same for publishers – unintended latency and money left on the table. Moving all header bidders to async allows equal opportunity to submit bids which will increase competition and monetisation, allowing header bidding to live up to its potential.

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