Some tape drives support a seek ioctl which allows positioning of the tape drive at a particular block. This makes restore a lot quicker, since taper can calculate which block a file(s) lies on an advance straight to that block.
By default, taper assumes that zftape, and SCSI drives support this ioctl while removable,ftape and IDE don't.
You can test if your tape drive supports seek by running the Test can seek utility and changing the appropriate preference.