I wonder if people are assuming that someone (or a team of people) had to read each email to "check" it. Not really. If you know what to look for it's possible to "check" a document or documents that stretch to hundreds of pages, eg by searching for keywords, and that doesn't take a computer long at all. According to the Times this morning, many or most of the new emails were found to be duplicates of ones the FBI already had on record -- again, software exists to automate comparison quickly, so I can well imagine this task being done in a day or less, leaving maybe a few hundred "new" emails to sort through.
Not a difficult task at all in principle, then, to have cleared this latest batch of emails in just over a week.
Still leaves the question: "why was this revealed in the first place, before the investigation had properly started?"