Lazy load tab

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Use Lazy load - toggle to enable or disable lazy load feature which lets ads to be rendered right at the moment when they appear in the visitor’s visable area. (It’s available only if Multi Request Architecture is active) 
Lazy load setting can be configured via the Lazy load tab.

We can differentiate between four settings here. When lazy load is switched on, the Long Distance value is switched on by default:

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The other distance values are as follows:

Please note that the values are entered individually here for Custom. As soon as the entered value differs from Long, Medium or Short, the field changes to Custom.

The lazy load settings can be specified for each publisher.

1. Fetch margin (in percentage of viewport) - how fast (how early before being in viewport) an ad should be fetched to an ad slot (ad request should be made)

2. Render margin (in percentage of viewport) - how fast (how early before being in viewport) an ad should be displayed (creative should be displayed)

3. Mobile scaling - multiplier of Render and Fetch margins for mobile devices

A very good simulation of the following drawings: Lazy Loading  |  Google Publisher Tag  |  Google for Developers

SRA (Single Request Architecture)

The settings for fetch margin are completely ignored

MRA (Multi Request Architecture) - required if Lazy Load is active