It is possible that the default block size was set to a much lower value. Check the setting using Admin > Devices. A block size of 120 offers excellent performance.
There are two types of block sizes. One is the high-level block size. It represents the number of 512 byte blocks sent to the tape drive in one write operation. The second is the low-level SCSI block size which only applies when using a SCSI tape drive. It represents the number of bytes the tape drive head uses. This should be 512 for quarter inch tape drives, 1024 for DAT drives, and 0 (variable) for every other type of tape drive. After any change in block size, test the tape. If it detects a problem with the specified block size, it suggests the proper block size to use.