Oof, xz is slow on elspeth. I wonder why it’s threading is broken.

  -T threads, --threads=threads  
         Specify the number of worker threads to use.  The actual number  
         of threads can be less than threads if using more threads would  
         exceed the memory usage limit.  
         Multithreaded compression and decompression are not implemented  
         yet, so this option has no effect for now.  
         As of writing (2010-09-27), it hasn't been decided if threads  
         will be used by default on multicore systems once support for  
         threading has been implemented.  Comments are welcome.  The com‐  
         plicating factor is that using many threads will increase the  
         memory usage dramatically.  Note that if multithreading will be  
         the default, it will probably be done so that single-threaded  
         and multithreaded modes produce the same output, so compression  
         ratio won't be significantly affected if threading will be  
         enabled by default.

the xz(1) manpage

Oh, I see.