Nowhere have I read anyone interviewing, talking to, etc, people who do exactly this kind of thing all day - attorneys. This process is literally what electronic discovery IS.
As an attorney who works on these types of issues, 650k emails is a trivially small corpus. Even if they were all responsive, and no duplicates, I would expect a decently-sized team to take about ten days to go through that number of emails. Of course, realistically, in most cases you are going to filter that down by 90% or more before a human being looks over a single email... depending on the process and work hours, a single person can easily look at 1k emails a day, and that's when you're doing legal analysis on them... if all you're doing is figuring out "Bullshit/not bullshit" and kicking it upstairs for further analysis 2k emails is not unreasonable for a 12 hour shift. Although it wouldn't be fun to be doing...
But still, at 100 emails/hour that's 6500 man hours of work if you want a human being to comb through every single email, or a hundred people working one 65-hour workweek, which even gives Comey the luxury of a full day to set up the process.
So when you read some idiot saying that it's not possible to have looked through that many emails... yes, yes it certainly is. A big document review team will comb through a million documents a week or more, and that includes doing legal analysis on them as well.