The problem is most clearly visible in the difference between https://docs.rs/tokio/1.36.0/tokio/runtime/struct.Runtime.html#method.spawn https://docs.rs/tokio

The problem is most clearly visible in the difference between https://docs.rs/tokio/1.36.0/tokio/runtime/struct.Runtime.html#method.spawn https://docs.rs/tokio/1.36.0/tokio/runtime/struct.Runtime.ht... https://docs.rs/async-executor/latest/async_executor/struct.LocalExecutor.html#method.spawn https://docs.rs/async-executor/latest/async_executor/struct.... Those Send + 'static bounds have really big impact. Multithread is definitely not just "additional throughput for free", nor is the change between single to multi just one line of code.