Not just can be, it usually is faster. At least, once it's been JITed. We just ran some XML serialization/deserialization tests, and the java implementation was much faster than the C++ one...eventually.
What about pure numerical stuff? I'm wondering how say, a Java implementation of FFT would fair against the FFTW library (coded in C).
Of course you could make a native call to the FFTW library (MATLAB does this) through Java, but thats missing the point.