Server islands are a solution to the waiting for fully dynamic server side rendering vs. Single Page App.
Server islands serve a prerendered HTML file, which includes islands. These islands then request more contents from the server. Yet, the islands will not block the initial response from the server. Therefore, the website is not blocked by the rendering of the islands.
Server islands are not the same as lazy loading components. Rather, they can be imagined as lazy loading + requiring dynamic data from the server.
A website using server islands can be served from a CDN.
Astro has capabilities for server islands.