E-Mail Migration API Seems to Slow Down with Multiple Accounts with Large Mail Stores

[Bob]

When we have maximum users migrating at once (currently limited to five admin accounts that run the API for end-users because of the preliminary maildir conversion causing high load-averages on the server), and they have a lot of e-mail (700 Meg to 2 Gig worth), the whole operation seems to, and in looking at the migration log/summary, slows down for all five. Also, there seems to be a tipping point at which the throttling gets intense, at about 500 to 600 Meg. We don't have a lot of users, at this point about 1900 left of the ~3500 (we've been doing early adopters for a few months), but many of the users have 1 Gig+ mail store each. Has anyone run across this scenario and have any suggestions?  High load average on the server because of the first part of the migration notwithstanding, would increasing the number of API running accounts help? Thanks, -Bob