I'm confused: Matlab uses MKL, Armadillo can be configured to work with MKL if you have it, and Julia supports MKL, and NumPy too. So if everything uses the same hardware vendor libraries, I'm not sure if that's where your actual speedup will come from. Also, MKL syntax itself is not particularly fun compared with Matlab/NumPy/Julia/etc.
Once your language allows you to use MKL and its equivalents, your code will use every arithmetic unit of the CPU in almost every cycle and therefore performance-wise there is no difference which language it is. So it's all down to which language offers you better library support and nicer syntax.
Hmm. If it we're true that speed is independent from language once mkl is installed, wouldn't you expect there to be no observed performance differences? And yet there are large measurable ones for reasonable tasks even with mkl.
