Unfortunately the first post is always loaded together with a discussion. It's used by Flarum itself for pinned discussions and search results previews.
Some extensions also rely on that value to show discussion thumbnails or other customized excerpts.
It should be possible to create an extension that removes that data if it's impacting performance too much. But you might lose some features from various extensions that rely on the
firstPost discussion relationship.
Another option would be for an extension to truncate the HTML itself before being returned by the API. It has some challenges because the HTML needs to be cut in a way that remains valid, and additional logic must be added to the single page app to ensure the full post is reloaded with complete HTML once the discussion is opened from the list.