<p>mlpack's k-means does not use OpenMP at this point in time.  So yes, what you're seeing is normal.  But do keep in mind that mlpack's k-means implementation is far more flexible, allowing you to use five or six different algorithms---one of which will probably be faster than the default O(kN) algorithm.  (Try it;  for your dataset I'd expect Hamerly's algorithm or the single-tree Pelleg-Moore algorithm to be fastest.)</p>

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