Fast Fetch rendering reduces latency, increases viewability, and improves the end user experience of your ads. All Ad Manager tags on AMP pages support Fast Fetch.
With Fast Fetch, ads on AMP pages are fetched asynchronously, and only render when they are likely to be viewed by users. Ad requests occur earlier in a page's life cycle (AMPHTML ads are rendered immediately, and non-AMPHTML ads are delayed). Only ads in the first three viewports on the page are rendered, and subsequent ads are rendered as the user's viewport gets within three lengths of the ad slot.
Real-Time Config
Real-Time Config (RTC) is a feature of Fast Fetch for AMP that enables publishers to augment ad requests with first- and third-party targeting information that’s retrieved at runtime. RTC allows up to 5 callouts to targeting servers (endpoints) for each individual ad slot. These endpoints may return a JSON document with targeting data, which the Ad Manager tag appends to the query for the ad slot to target ads.
To use RTC with Ad Manager, specify the rtc-config attribute for each <amp-ad> element where it should apply.
Learn more about RTC: