Reading Wiener's own emails, those unrelated to his wife Huma or Hillary, is unlikely to be productive. Therefore, the FBI is going to filter those 650,000 Wiener emails to get at those emails that were also sent to/from Hillary and Huma.
That's easy for automated tools to do. Just search the From: and To: fields for email addresses known to be used by Hillary and associates. For example, search for firstname.lastname@example.org (Hillary's current email address) and email@example.com(Huma Abedin's current email).
...This doesn't mean that there's not something interesting (or even evidence of a crime) in the emails. However, it does mean that the emails that got found weren't to or from Clinton.
This is likely to filter down the emails to a manageable few thousand.
Next, filter the emails for ones already in the FBI's possession. The easiest way is using the Message-ID: header. It's a random value created for every email. If a Weiner email has the same Message-ID as an email already retrieved from Huma and Hillary, then the FBI can ignore it.
This is then like to reduce the number of emails need for review to less than a thousand, or less than 100, or even all the way down to zero. And indeed, that's what NBC news is reporting