A Google employee commented above about the design reasoning behind this, and it's worth reading and understanding the linked GitHub discussion:
> It looks to me like the design for this was https://github.com/ampproject/amphtml/issues/3133
To be clear, I'm not defending Google here, but I just want to point out that it's possible to be "evil" (for whatever that means) without being malicious. That is, I don't think any Google engineer was thinking "Haha! We'll slow down all those non-AMP ads deliberately!" The thinking seems to have been "We want to be somewhat 'backwards compatible' to allow non-AMP ads, but we don't want those ads to kill AMP's performance benefits, so we'll still load them but only after the AMP content has loaded and rendered."
However, this is exactly the mindset that I hate about AMP, and it drives me a little nuts when I see Googlers defending it. I get that your intentions are good, but don't you realize that it's not an accident that the primary beneficiary of AMP is Google, e.g. by keeping users in the SERP instead of going off to sites where the content is hosted, and by making everything ideal for the ad network you own? Googlers have to come to realize that essentially all of their engineering decisions are constrained by what makes more money for Google, not what is best for users. Sure, faster page loads are great, but do you honestly think Google would have supported a tech that made page loads faster ig it didn't cause people to look at more Google ads? I mean, if Google really supported what was best for users, they'd support more privacy controls a la what is coming in iOS Safari, but of course they don't because that's bad for their business.
I hate AMP and hope it dies, but I don't hate it because I think there aren't some user benefits to it, I hate it because it is a further attempt by a single company to own even more of the Internet.