Since April 25th, Google enabled lazy loading for publishers using the legacy Ad Exchange tag (publishers using the Google Publisher Tag are not affected).
According to Google, lazy loading (or smart loading) enables pages to load faster, reducing latency and CPU consumption by pausing ad requests for any slot that is far from the user’s viewport. Those ad requests are subsequently sent when the slot is closer to the viewport and more likely to become viewable.
In the long term and in aggregate, Google expects that publishers will see a neutral revenue impact due to a reduction in non-viewable impressions, and a subsequent increase in viewability rates.
Publishers that have pages with a large number of down-screen, low-viewability slots, may see a decrease in queries, impressions, and revenue.
Lazy loading can affect viewability in two ways:
In cases where a user didn’t scroll down, for example, the Below-the-fold ad slots never loaded, fewer ad requests are made. However, if the ad size is too large or loads too late, for example, when the user scrolls down faster than the ad loads, the user won’t be able to view the ad.