The whole "java is slow" argument is outdated by many years. It only survives as a joke among programmers, and as a trope on the internet for people to hate on something for no good reason, because hating on things for no good reason is "cool". With the advancements in compilers, CPU architectures, and the language itself, the relative speed of Java versus C++, or C#, or any modern widely-used programming language is at most academical, and almost always circumstantial. I wouldn't consider myself a Java expert, although I know enough about Java, C++ (and half a dozen other languages) to say that a program is as fast as its programmer, not as its language. There are applications which run faster in Java than an equivalent implementation in C. (mostly because of lack of pointer arithmetic and associated memory aliasing problems, also a little more compiler-friendly implementation of generics vs. templates)
You should choose a programming language based on its native features, expression power, ease of development, available libraries, other software you'll be interfacing with, etc., not because of "speed" - unless you really know what you're talking about - which means talking about a specific problem, having written an implementation in both languages, optimized the crap out of both, and measured profiling data.