If you read documentation around Rust Async and Tokio, you'll find proper explanation why CPU intensive parts should not be part of async stack, how to use primitives efficiently (like std::sync::Mutex in async blocks), how to glue sync and async code.

A lot of code doesn't follow there guidelines because they don't care about efficiency and don't need it. But there are numerous projects who care about performance and efficiency, and realize the pitfalls once code runs in production (ScyllaDB is one example).

LLMs don't help as well, generating everything async up to the main, using wrong primitives and not properly designing the system.