This is super important. I don't think ad networks are doing as much towards JS performance (or even image resizing!) that they could be doing. Audit the ads being added to your site. Defer the ad JS as much as you can to later n the load sequence.
If you find really horrible behaviour - talk to the ad provider. If possible switch to one that optimizes content. I'd love to think that the market can push ad providers to optimize their content - but we will see.
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