First of all, you need to determine if there is any buffer related problems. Enable the bptm logs and scan for any delays....eg...waited for full buffers...waited for empty buffer...etc. If you discover any anormalies then you need to fine tune the buffers e.g
bptm log: Write buffer information
16:10:13 <2> write_data: waited for full buffer 4111 times, delayed 8701 times
This indicates less data is being received from the network/disk than the tape can handle. You can reduced the buffer size (SIZE_DATA_BUFFERS) but make sure not to reduce below the default values. For windows system this is normally 64K
Also remember the buffer sizes are set in bytes and not Kbytes! If you are not sure, read the manuals first.
Remember, this settings may also affect your restore time.