Even as you observe that only one CPU resource is being fully utilized, remember to consider all of the various resources that could be sources of contention.   Real memory (i.e. actual RAM chips) should be fully utilized if possible, but not to the extent that significant virtual-memory paging begins to occur.   (Don’t forget that a fair amount of RAM is, and should be, used for file buffers.)   The practical amount of disk-I/O throughput that your system is capable of, is always a fundamental constraint.   It is quite common that a system that is operating at-or-near its actual pragmatic capacity to do work is “barely using” its CPU resources, simply because they are so incredibly faster than everything else in the machine.