Threads aren't that slow on Linux. The main advantage of M:N threading as implemented in Go over 1:1 is that spawning is fast and doesn't use much memory, because you can avoid the syscall and only a small (initial) stack is required. Rust can't do the latter because it's not GC'd.
Even if it could, many real-world servers actually do non-trivial work in their threads, so the cost of spawning a thread is dwarfed by the actual work the thread ends up doing. There are serious drawbacks to M:N: complexity, fairness, problems in interoperability with the 1:1 world (including essentially unavoidable performance problems with the FFI), etc.
Even if it could, many real-world servers actually do non-trivial work in their threads, so the cost of spawning a thread is dwarfed by the actual work the thread ends up doing. There are serious drawbacks to M:N: complexity, fairness, problems in interoperability with the 1:1 world (including essentially unavoidable performance problems with the FFI), etc.