If you're dissatisfied with objc_msgSend performance, you can write C or C++ seamlessly - or use that other language that's partially designed to address the dynamic dispatch nightmare.
If you're dissatisfied with JavaScript call performance, you have no choice.
That's not what I said. What I said was that JavaScript would continue to be the most important client-side language on the Web. It's great that Web Assembly gives developers alternatives, and I hope it gets used to write games and to accelerate performance-critical parts of apps. I also hope that it allows people who don't want to code front-end logic in JS to deploy alternatives--I'm not even passing judgment on whether you should use JS (although I think choosing C++ over JS for high-level front-end app logic is a very poor decision). I'm just nearly certain most code will continue to be written in JS.
If you're dissatisfied with JavaScript call performance, you have no choice.