Google’s AMP (Accelerated Mobile Pages) team has launched a new feature called “Render on Idle” now available for publishers using the DoubleClick AMP ad tag, and any ad network choosing to implement Fast Fetch can also take advantage of it. 

From AMP Github repo -

Slots not marked with data-loading-strategy attribute that are more than 3 viewports but less than 12 from current location are allowed to render when the AMP scheduler is idle. The result is an increase in impressions with a much smaller increase in viewable impressions and clicks.

Learn official announcement here and visit Github to learn more about “Render on Idle”