We were noticing super slow load times for our wp-admin pages. After lots and lots of debug statements we found that the culprit was FeedWordPress. It has a registered shutdown event email_diagnostic_log which apparently sends emails to the admins of the blog if there are errors.
For the recipient list it wants to use an option called "feedwordpress_diagnostics_log_recipients". I can't see how that ever gets set... so for us it is NULL. This means that the code then goes out to figure out who are all the admins for this site. That takes quite a long time when you have lots and lots of users.
The saddest thing is that the next step it takes is to decide if it should even be sending errors at this time. It has a nice way of only running this code once a day, but unfortunately the most expensive chunk of code (in our case) is located OUTSIDE of this check.
If the authors could move the building of recipients inside the time-checked loop, this would be much more efficient.