Hydration is expensive for two reasons:

The frameworks have to download all of the component code associated with the current page.

The frameworks have to execute the templates associated with the components on the page to rebuild the listener location and the internal component tree.

A good mental model is that Qwik applications at any point of their lifecycle can be serialized and moved to a different VM instance (server to browser). There, the application simply resumes where the serialization stopped. No hydration is required. This is why we say that Qwik applications don't hydrate; they resume.