In the context of a browser, I doubt that the standard 20-50% difference applies. That difference is what you see when measuring JVM code against optimized C++. In the browser world, that kind of pure speed doesn't happen because achieving security means layering all of the bounds checking and such back into the code, erasing one of the major reasons why the JVM code is slower.

In C/C++ you have the option of not performing those kinds of checks to get speed. In a secure browser, you don't want to take that risk unless it's absolutely necessary.