How many drives do you have?  You can set the migration process to more
than one which will allow you to write to multiple tapes at once.  I will a
lot of times set up administrative schedules to change the migration levels
so that while clients are backing up to disk, migration will not be
running.  When I know that all of the backups are complete then change to
lower values that will kick off migration.  You can have client backups go
directly to tape which is usually faster but this will only works well if
you don't have more clients than drives because each client will tie up a
drive.  We often will have large (oracle, db2, etc.) database backups go
directly to tape.