Web pages are far more static than users, your adserver/ML/analysis tool need only analyse the page once (not including updates for content changes) and then pass this information straight on to ad buyers.

Even if advertisers analysed the page content on seeing it, rather than the publisher, it would still be net faster than doing lookups/analysis per adserve (as you would when you target a user).

I don't think it would be hard at all to build something which analysed a page and made an adserving decision with a lower "time to ad display" than a similar process that used user data.