I normally compress using tar zcvf and decompress using tar zxvf (using gzip due to habit).
I've recently gotten a quad core CPU with hyperthreading, so I have 8 logical cores, and I notice that many of the cores are unused during compression/decompression.
Is there any way I can utilize the unused cores to make it faster?
You can use pigz instead of gzip, which does gzip compression on multiple cores. Instead of using the -z option, you would pipe it through pigz:
tar cf - paths-to-archive | pigz > archive.tar.gz
By default, pigz uses the number of available cores, or eight if it could not query that. You can ask for more with -p n, e.g. -p 32. pigz has the same options as gzip, so you can request better compression with -9. E.g.
tar cf - paths-to-archive | pigz -9 -p 32 > archive.tar.gz