How many cores does your system have? If it's more than one, are you looking at the usage per-core or is this reported as a percentage of the entire machine cycles available?
You are actually using 100% of 1 core which is all NX is programmed to use, except in certain areas where it can do multithreaded tasks (NC processing, rendering, CAE, etc).
1 core at 100% of an 8 core machine is the 13% you are seeing.
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