As a bonus, the code is now statically typed
If JS is only half the speed of a compiled language like Rust, that shows remarkably optimized performance.
Things don't need to be "exceptionally" fast to be in the area where programming language doesn't really matter.
> Twice the performance on a server could let you handle twice as many concurrent sessions (and possible run half as many servers!)
Which might matter, or it might not. Very situational.
> People regularly fight tooth and nail to squeeze 10% performance boosts on critical tasks, doubling it would be incredible
That kind of task is a small fraction of tasks. And often you're best off using a library, which can often make its own language choices independent of yours.
The tradeoff is that Rust is lower-level, so it is harder to write. If performance were the only point of comparison for a language, then we'd all be using assembly. We choose to trade performance for productivity when we're able to.