You surely must be referring to Rust, the only multithreaded language with async-await in which data races aren't possible.
Rust is lovely and all, but is a bad example for the performance side of the argument, since in practice libraries usually have to decide on an async runtime, so in practice library users have to launch that runtime (usually Tokio) to execute the library's Futures.
Sure, but that’s a library limitation (no widespread common runtime interface that libraries such as Tokio implement), not a fundamental limitation of async.
Thread safety is also a lot easier to achieve in languages like C#, and then of course you have single-threaded environments like JS and Python.
Rust is lovely and all, but is a bad example for the performance side of the argument, since in practice libraries usually have to decide on an async runtime, so in practice library users have to launch that runtime (usually Tokio) to execute the library's Futures.