JavaScript really isn't that slow. JIT compilation can wind up faster than AOT compilation. And much of the APIs called by JavaScript is natively-implemented browser code. JavaScript is faster than C# yet people implement games in C# (not the engine cores, but that's a very similar situation to JS) and don't bat an eye.
> JavaScript is faster than C#
Are you joking? C# is in another leage, when we talk about performance.
> JavaScript is faster than C#
Apples to oranges, scripts need an entire browser/Interpeter framework underneath it to even function
See the mentioned game engine reference which closely parallels this.