Do you need outstanding core web vital scores to be included in Google Discover? The answer is no, at least John Mueller of Google said, "we don't have that connection documented anywhere."
John also said, "I'd be surprised if CWV were a requirement for Discover."
He said this on Mastodon when he was asked, "many people ask me if the site's loading speed must be high to enter Google Discover? Or should our urls be fine in CWV?"
In short, no, you do not need to have amazing core web vital scores to be included in Google Discover.
John did also link to the Google Discover content policies and the Get on Google Discover help documents.
Forum discussion at Mastodon.