> So a hypothetical switch to first-party/server side ads won't have a negative effect on small blogs, because they can simply chose Google as their ad partner, which will create the necessary solutions.

With client side ad serving the ad serving network can be very fast, because google, etc. provides the cdn for this.

If it is done on the server side then when loading the page the server will have to do additional remote network calls to retrieve the ads from google servers, so the page will be slower to respond. Also, smaller sites usually run on budget hosting which often limit resources and having to do additonal remote network fetches when assembling the page can very easily run into some server side limit.

So server side ad serving for little sites can have a significant impact on server response time which is not an issue with the current model.