In the pages directory, pages using server-rendering would show the browser loading spinner until getServerSideProps had finished, then render the React component for that page. This can be described as "all or nothing" data fetching. Either you had the entire data for your page, or none.
In the app directory, you have additional options to explore:
First, you can use loading.js to show an instant loading state from the server while streaming in the result from your data fetching function.
Second, you can move data fetching lower in the component tree to only block rendering for the parts of the page that need it. For example, moving data fetching to a specific component rather than fetching it at the root layout.
Whenever possible, it's best to fetch data in the segment that uses it. This also allows you to show a loading state for only the part of the page that is loading, and not the entire page.
Data Fetching without fetch()