The place where performance really matters is generally in the core learning algorithms. Since these are all written in C or Java, you can very easily get away

The place where performance really matters is generally in the core learning algorithms. Since these are all written in C or Java, you can very easily get away with programming the rest of the code in a higher level language. This is why Python is a feasible choice for machine learning at all. That said, Clojure is certainly a lot faster than Python for native code and the performance gap with Java is continuing to drop. It's also worth pointing out that you should not ignore C or C++ libraries just because your are running on the JVM. It's not very hard to interface to a core C library with JNI, although be warned, there is a slight trick to doing this via clojure rather than Java.