When using the Add to Archive dialog to add files to a .tbz2 archive, the following options are available:
Compression level: This lets you set the compression level used when adding the files to the archive. There are five compression levels available, ranging from Fastest (the quickest, but poorest compression level) to Best (which produces the highest level of compression but takes longer to archive).
Use multiple threads: If this option is on then Opus will use multiple threads when adding to the archive - on a multi-CPU machine this should make the compression process quicker.