I'm not sure that portability is a good reason to use Java in this case. Firstly, these trading firms probably have tight control over their server architectur

I'm not sure that portability is a good reason to use Java in this case. Firstly, these trading firms probably have tight control over their server architectures. Secondly, once you start using low-level Java idioms in order to optimize your code, you've started approaching the point where differences in performance between JVM implementations across platforms become noticeable.