Does mirroring slow down Backups?

Fellow Informix'ers, We have a database that uses Raw disks wit some blob chunks. This db is completely mirrored, and backups on this DB are really slow. There is another installation which is similar, except no mirroring. Backups at that site seem to be almost twice as fast. Does mirroring indeed slow down the backup (and the restore) process? Any and all responses appreciated. Thanx, Raj.