There's a reason your OS, your browser, your shell, your word processor, etc. are written in C and or C++ not to mention your JVM, python, perl, your favorite libraries, etc. It's not "momentum", it's "track record". Yes, you can shoot yourself in the foot with C/C++. Experts know this and manage not to maim themselves.
Java has improved ... a lot ... no doubt but the " 20-50%" is a tad hyperbolic. Yes, repeatedly calling the same function (who's internals are in C) with Java sometimes you can get to with 20%. And , of course, carefully crafted java routines can beat naive, sloppy C sometimes.
Back in the day, Java proponents promised us the Javagator [ http://www.wired.com/science/discoveries/news/1998/04/11458 ].
Don't hold your breath for a competitive Java browser. The speed and small foot print of C/C++ will continue to be edge giving it almost all the market share.