Hi,
the new default for Blocksize is 256K. Normally and if not configured in old environment, the blocksize is automatically set to 256K. If you need to backup HP-UX, stay with 256K, If you have a Windows environment, increase up to 512K. Please note: all media you want to use with the new blocksize must be reformatted, as the media header at the beginning of tape must be formatted using the new block size.
In addition I recommend to increase the Segment Size too (try 10000).
Both settings (together with an optimized and configured omnirc file) will increase overall performance of your backups.
Best regards
Daniel