Since swift is released as open source it would be interesting to compare swift performances. I found the interesting project http://github.com/andreaferretti/kmeans that compares the execution time of the KMeans algorithm on several languages.
From the results it looks like that the execution time is comparable to python and it is pretty distant from the top.
http://github.com/andreaferretti/kmeans/blob/master/results Results ------- nim (0.11.3) 148 ms java (1.7.0_76) 183 ms .... python (2.7.6) 10632 ms pharo (3) 11170 ms swift (2.2-dev) 11391 ms perl (5.20.2) 15680 ms rubinius (2.5.2) 20878 ms ruby (1.9.3p484) 24819 ms