If you output a lot of alternate blocks within your ODD, e.g. for places, people or terms, it can slow down rendering of the text, in particular, if the content to be shown as alternate requires further processing or is loaded from an external database system via an API.
To help with this, the pb-popover webcomponent allows popup content to be loaded asynchronously. pb-popover is the default component which is output for <model behaviour="alternate">.