I've never heard anybody say that Java is that fast. It can be in very specific situations. Actually 2 - 3 times as fast as C++ is pretty awesome, especially since that is hardly low level Java, rather quite abstract. I suspect low level Java could get it down to a flat 2x in this case. For a huge range of uses 2x as fast is effectively as fast as C, especially since taking advantage of multicore / parallelized operations is significantly easier (in a cross platform manner) than it is in C/C++.

In my experience Java is usually at most 50% worse than C++. This is only the case though when I write Java with minimal use of objects (so no generics for instance). Also this is in comparison to C++ code that's not too micro-optimized (so mostly idiomatic) and that is not compiled via profile-guided optimization for instance.