2-3x slower? I guess it depends on what you're doing. For a lot of code I've dealt with the differences in performance have been factional.

This doesn't make much sense, unless:

1) You wrote that code in BOTH Java and C++, optimized both, and compared their running times.

2) Said code was CPU bound. For heavy IO bound code you could even use TCL and get "fractional" differences.

Only computation-heavy code will show a difference. And the disparity is going to vary a lot. Some code will be easy for Java's optimizer and won't give C++ any advantage.

Some will even put C++ at a disadvantage.

Out of curiosity any such examples ?

Of course there are, and it only takes a few seconds with Google to find them. Here's one: the frannkuch-redux benchmark at http://benchmarksgame.alioth.debian.org/u64q/java.php gives Java a >20% CPU efficiency advantage over C++.

That compares a Java program written to use multiple cores with a C++ program not written to use multiple cores; because the C++ programs written to use multiple cores did not compile.

See http://benchmarksgame.alioth.debian.org/u64q/performance.php...

It is very good. But it's not quite fair to compare it to, say ruby or PHP, because it still requires a compilation step, and it's still pretty demanding on the programmer.