Performance in web apps is not a goalpost for everybody. While there's certainly web apps that are problematic in terms of performance, you're unlikely to be running at 100% CPU the whole time, so it's not like using C will be the right solution.

I can see how C/C++ (and, well, more likely Rust) will become a de-facto solution for when you need performance or some type of robustness to your apps (ie probably the right thing for Figma). But the potential performance improvement might be negligible on most makes, and unlikely to offset the fact that you'd be working off a completely different stack making things like debugging and inspecting much harder.

What I think is likely is that using WebAssembly will allow new types of web apps to become popular. Things like video encoders, image editors, etc. Things that you just wouldn't do with JS anyway, at least not in the heavy parts. Thing of a standard web app that uses ffmpeg on the background.