Almost certainly you are being limited by media write speed. With a single HD you are probably talking under 100MB/s at best and modern CPUs are capable of decompressing way faster than that. Even 5 years ago CPUs weren't getting maxed out.
If you want to see better CPU utilisation, then you really need to get yourself some faster storage, i.e. extracting from one unfragmented RAID0 array (Velociraptor/SSD) to another.
65MB/s read speed is all well and good, but that's not the bottleneck, it needs to write as well, and reading and writing from the same drive will slow things down even further (it's a mechanical device and as such isn't 'full duplex')