"Best" blocksize for writing backup to LTO tape drive

I'd like guidance from the more experienced on what the best block size to
use is.  I let it default to 32768 and the backup is running quite slowly
(though this is running under valgrind).

I'm using bzip2 (9) on an Atom D525 which might be the cause of the problem.
Should I instead use no software compression and use hardware?


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