But what about the ads?

AMP by default, loads ads after the content and generally lazily. 

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One of the core elements of AMP ( Accelerated Mobile Pages )  is that is ships with a validator, that checks compliance of AMP documents with certain rules. The idea is: If the rules are met, then the document will load fast. The ruleset we designed for AMP has a broad set of document types and use cases in mind. Ad creatives, on the other hand, are a much more focused use case, and hence they do not actually require the full breadth of functionality that AMP exposes. For this reason, AMP for ads carefully picks out the aspects of AMP that are required to build ad creatives.  

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Ads often come with their own set of measurement tools to collect important information such as whether an ad was actually seen by a user. This substantially increases ad payload size, initial compilation and execution time, battery usage and other aspects of runtime performance.

AMP, through its established `amp-analytics` mechanism, already ships with all the code to perform these measurements. It is vendor neutral and supports a wide range of metrics.
https://amphtml.wordpress.com/2016/07/19/but-what-about-the-ads/