Recently, I wanted to maintain a map of routes to futures so that multiple in-flight requests could await the same future while it fetches from disk.

The Javascript version took about one minute.

Using Rust doesn't give you free performance wins, especially not when working with async code where lifetimes actually get hard. Any time you're carrying around your own event loop / cpu pool while trying to avoid blocking requests, that's also another bombshell in itself. Similar challenges when using something like NIO/Netty, especially towards the beginning.

Add things like sinking a stream of lazy futures into a response and it can be very hard to understand what your actual performance looks like and where things will block, especially with parallel requests. The only way you could say it's a free lunch is if you're a Rust expert who has done it before, which doesn't apply if you're considering Rust vs JS/TS anyways.

Not to say nobody out there should be writing Rust, that would be silly. But someone upstream is seriously asking what advantages JS could have over Rust and difficulty is one of them. And it's not a small one.