I don't think for a second that async Rust should be picked for performance reasons.
You get a feeling for what is a good use of async and bad use of async relatively easily these days as the ecosystem is maturing.
How do you know if what is best doesn't change as the project you're working on progresses and your manager tosses in new requirements?
I'd say better pick a technique (or even language) that works all the time.
Choosing performance as your #1 priority is often a bad idea as it gets you into a straight-jacket from the start, making everything else much more difficult and slows down development to a crawl. Unless you're developing an OS kernel perhaps. Computers are fast enough these days, let them do part of the work for you! And you can always write a faster version of your software when there is a demand for it.