lbzip2(1) -d -n 32 my_file.bz2
parallel bzip2 utility
-d, --decompress
       Force decompression over the mode of operation selected by the invocation name.
       Set  the number of (de)compressor threads to WTHRS.  If this option is not specified, lbzip2 tries
       to query the system for the number of online processors (if both the compilation  environment  and
       the  execution  environment support that), or exits with an error (if it's unable to determine the
       number of processors online).
source manpages: lbzip2