Both JavaScript and WebAssembly have the same peak performance. They are equally fast. But it is much easier to stay on the fast path with WebAssembly than it is with JavaScript. Or the other way around. It is way too easy sometimes to unknowingly and unintentionally end up in a slow path in your JavaScript engine than it is in the WebAssembly engine.
Using the language du jour on the browser will typically require the download of a massive runtime unless something like C/C++/Rust is used and those tend to inflate development time. So using WebAssembly has to be seen as an optimization once things stabilize.
In this case performance is about using the available resources to the best effect - JavaScript on the browser is (for some time to come only) part of the whole picture.
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That attitude simply ignores the realities on the web. The browser already has a runtime for JavaScript so you don't have to ship one.
WebAssembly for Web Developers (Google I/O ’19):
Also Replacing a hot path in your app's JavaScript with WebAssembly.
Using the language du jour on the browser will typically require the download of a massive runtime unless something like C/C++/Rust is used and those tend to inflate development time. So using WebAssembly has to be seen as an optimization once things stabilize.
In this case performance is about using the available resources to the best effect - JavaScript on the browser is (for some time to come only) part of the whole picture.