Generally, slowness in the initial migration (which is normally manifested by a delay in the initial messages starting to migrate) is because the migration wizard is waiting for the AD to replicate the directory information.
When the migration wizard create sa mailbox (or NT Account) it can take up to 15 min before the AD has replicated this information. As such, the migration wizard will sit and wait to start a migration. It will retry every min for up to 15 min. Typically this is what people are experiencing when they mention slow migrations.
However, if you batch (ie: Run 10+ users) when you migrate, you will only see the issue on the first 1 or 2 mailboxes, since by the time the process has completed with the first few mailboxes, AD has completed its replica.