Calling show Percabeth rushed is some funny because people act like they're already dating. We get to see them develop a crush (which is canon that they had at 12) but most importantly they're building a friendship.

We're also not in Percy's POV so ofc we can see how Annabeth acts around him objectively and not how, my sweet oblivious child, Percy interprets it. Also how he acts.

Readers also know what's to come so ofc we read into their dynamic a bit differently than non-readers. But nothing about their current dynamics is inherently romantic.

They're slow burn not because they're at odds with each other or because they don't like each other. They're a slow burn because YEARNING, takes them a while to process and accept those feelings and the whole you know, fucked up circumstances they live through so they have other priorities.