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It's referring to things like analytics, hotjar etc.

Optimisation as in conversion optimisation.

Imo, a lot of this stuff ends up being just as bad.

Considering many of those read first party auth tokens they are particularly bad.

Yes, when one website relies on the fact that you're logged in somewhere else.

Thing easily being able to add embedded Youtube videos to your watchlist, adding favorite articles to Pocket, being able to pay with 1 click using a payment method saved somewhere else, and so on.

I'm thinking that there are better ways to do this than relying on potentially fragile or hackable cross-site cookies. Certainly for payment, I'd prefer that communication to be handled server-side.

Optimize you buying things.

CDNs maybe? Back when people used jQuery, an enormous amount of traffic went to Google, ripe for tracking, and you got the small benefit of not having to download the same version of jQuery so often.